Because the original version of this guide is pretty much entirely credited to me, there's not a whole lot of fact checking or error hunting (I've fixed plenty of spelling mistakes already, and even a few tidbits about what certain classes are capable of, which is even less okay to be wrong about since this is an MvM guide and not a spelling guide). I'm working on this! ...but not much, because my word is divine and you should do what I say *cough*.

To make matters worse, the guide covers so many topics, there's no way a team could possibly quality control it, let alone an egotistical scientist who is (rightfully) completely full of himself.

So here's why you should read it anyway. I'm going to spam some qualifications at you! Yes, the thing that makes this guide unique is that the guy who wrote it actually uses many objective tools in his analysis of the game despite his flagrant and unashamed holier-than-thou (which I am) stance.

I play lots of Mann vs. Machine. I think it's the best thing to happen to TF2 because it actually puts the "Team" in "TEAM Fortress" unlike any other mode I've played. It also has the upgrade purchasing that simulates that leveling up experience that makes games like Castlevania so fun.

After playing it for so long I decided to convert my server into an MvM server, and later I took the MvM Stats plugin popular on many custom MvM servers and made great improvements to it - to my knowledge only my server uses the advanced, far more accurate version at the time of this writing.

With tools like my server and its statistical reports, PV HUD for seeing exact HP amounts of enemies' remaining health, the game's own combat text - batching and seeing damage numbers to begin with - as well as checking the score of the players in game when a friend is on the same server as me, I've made many observations that seem to be relatively unknown to the MvM community for whatever reason. The biggest issue seems to be an irksome conservativism enforced by bad players afraid of losing and not getting any loot who are less concerned with having fun and improving.

I've used such tools for many (victorious, mind you - I base my knowledge off of what has actually worked) matches of MvM as various classes, using various loadouts and doing so for each of the (at present) 23 missions - including mvm_coaltown_event. My server runs Caliginous Caper in particular, we only play with 4000$ (just as the wave was originally) and we've beaten it with only 4 people - 3 of which were Pyros. The group was composed of good players, but more interestingly, Pyro is just a strong class for MvM and Heavy is surprisingly lacklustre by comparison.

Hopefully this guide will embellish examples like this into a general mindset of investigating what works on your own and dominating Mann vs. Machine with skill and knowledge. You know, instead of simply mimicking teams that may well have been poor.

I'm more hopeful and less optimistic that this guide will indeed have the power to give a group of utter new2mvms, both brand new and those who think they know MvM but really don't, the ability to be matched with up to 5 other complete strangers of ANY skill level and still be able to take on ANY mission.

Build in the path.
Behind the first corner for normal waves, in the actual path to block Giant Scouts.

Corner camping is key - they come around the corner unprepared, you point blank them for maximum ramp up before they can react.
Bot stew at your feet, money is easily collected, they get hurt a lot and you get hurt very little.
This works even without a dispenser, but if there's an Engineer, there should be a dispenser at the first corner.
Sentry too, usually, although Sentry can be moved back to a tighter chokepoint for handling Giant Scouts.

Stand on top of things to avoid Spies. Build against a wall if there's lots of Spies so they can only approach from a limited number of sides.

If you're using a class with Penetration, stand on even footing and use it.

Sniper is an exception as the Sniper can use their Penetration effectively even while elevated due to their main targets being Giants.
Indeed, Sniper should be elevated, safely out of the way and preferably have a health and ammo pack near them.

Don't pop Uber Medics. Get a Medic Killer and let them handle it.
Conversely, if you have no Medic Killer, pop the Medics ASAP. If they're Quick-Fix Medics, just kill them already.

Using Crits on Giants and dangerous bots is more important than using Crits on the tank.
Make it safe to fight the tank before engaging it, THEN go buy more Crit canteens and waste it.
Do NOT melee the tank. Exceptions will be detailed in the tank section.
Medic should share Crit canteens if possible.

If a Pyro is trying to reset the bomb, for the love of all that is good, move your sentry or Wrangle it and DO NOT kill the reset target.

In fact, in general you should NOT steal kills in these stupid situations:

• Medic is trying to build Uber off of a Sniper
  Some Snipers will take 3 hits to kill instead of the usual 2 (actually, usual 1, because Ubersaw's crit rate is ridiculous).
  Why would you risk robbing your Medic of 75% uber by killing just one Sniper bot?
  
• Medic is trying to build Uber off of a Sentry Buster that is easily killed
  Sentry Busters should NOT be killed. It messes with the Engineer's pacing and more importantly it's a waste of time.
  Let the Medic Ubersaw it and move on.
  
• Demoman is trying to start a Crit chain.
  Demoman is probably doing something wrong if they're trying to use Crits On Kill, but if they are, don't hinder them anymore than they already have by stealing their kills.
  ESPECIALLY if they picked Demoknight, which is a horrible idea.
  If it becomes that much of a problem, just kick them, but until then, don't steal their kills.
  Otherwise it's your fault and they should kick YOU.
  
• Spy is trying to start a Crit chain to help with a tank.
  There's really no excuse for this. You have a powerful damage dealing, money grabbing, Uber Medic killing machine on your team.
  Don't interfere with their only way of doing sensible damage to a tank.
  
• Hell, if any of your team mates has Crits On Kill, don't steal ANY of their attempted kills with their ♥♥♥ weapon. Ever.
  It's really not that hard to let go of M1. You have no excuse. You're just bad.
  
• Sniper is trying to build Heads or Focus to prepare for a nasty part of the wave.
  This is probably hard to spot, so if your Sniper seems to be screwing around and doesn't speak up, ask them if they're having trouble using their weapon's ability.
  In the rare chance they say "quit stealing my kills" instead of making up some excuse for having horrible aim, help them out instead of kicking them.
  Of course, if they're using a weapon like the Machina that has no bonus for aiming well, they're using a horrible weapon with low utility anyway and should probably be kicked.
  
• Screw it, let's just say this: If someone asks you not to steal their kills and cites one of their (likely extremely powerful) abilities as the reason, listen to them and do what they ask.
  MvM has insane rewards for On Kill abilities that should be capitalized on. Use them.

Damage Per Second is important. Increasing this ratio as much as possible is ideal.
It is not directly important, however - the point of Damage Per Second is to increase Damage Per Metre.
Damage Per Metre is the amount of damage done over a given distance of the bomb's path toward the goal.
After considering set backs (or forwards, if you mess up) of the bomb by things such as airblasts and explosions, imagine the total distance the bomb must travel to the hole as one long line.
You win if you deal a great enough Damge Per Metre to the bots traveling that line.
The reason bomb resetting is so important is because it greatly extends that line, effectively simulating a massive DPM increase in a single moment.
As a corollary, tank busting is important because there is no way to increase DPM against a tank except to increase raw DPS.

Grab it. If you absolutely can't, call it out for the money grabber. This will usually be a Scout.
If it's a Spy, don't complain if he doesn't get A+. It's not as easy.
Don't kill support classes if you don't need to. Don't spawn money in stupid places.
Don't pick a fight with support classes and die when there's only one Giant left to kill to win.
Don't complain at your team for grabbing money if you're the Scout.
Missing money is not worth the risk, but missing health is.
The enemies spawn in subwaves. There WILL be time to stop and grab money. Do so.
Exception: if you see your Scout and that he's doing his job, don't waste time trying to do it for them.

If they're to be bought, it's usually in a mission with many 100% crit enemies (Bigrock and Wave 666 come to mind), so Crit Resistance should be first.

Resistances shouldn't have money wasted on them by classes that are "safe".

This would be Scout (use Ubers), Soldier (to an extent), Demoman (don't go Knight please), Engineer (just hide behind your Sentry - although Blast Resistance can help), Medic (again, use Ubers), Sniper and Spy.

Pretty much everything except Pyro or Heavy.

A Heavy with good resistances can tank everything and is thus more efficient than a Heavy with offensive upgrades.

In missions without loads of 100% crit enemies, it's generally better for a Heavy to only buy Crit canteens and just dump crits into everything.

Otherwise they should just buy Resistances (again, Bigrock is a notable example, as is Wave 666).

Pyro can possibly corner camp to survive without Resistances and should be using Ubers to properly Phlogistinate, but once offensive upgrades are solid, Resistances may be the way to go.

Definitely so in Wave 666.

In Decoy, normal Coaltown and Mannworks, Resistances are generally a waste for everyone (though in the case of Desperation I had to buy Resistances as a Sniper because the team was so poor they basically left me to fight everything on my own - the enemies in the last wave also spawn such that it's hard to stay safe as one of the "safe" classes).

Health Regen is a nice complement to resistances in Wave 666 but is generally a waste of money otherwise.

I'll (mostly) let the numbers speak for themselves. Below is data I've collected using 15 seconds of each class using crits versus a 40000 HP tank.
The tank was never destroyed during these tests so they're fairly reliable.
Crit canteens were not used for some tests (Ubers were used instead for timing) because some classes (at least, Engineer) might not benefit from having Crit canteens on them.

Measurements are in damage per 15 seconds using 3 crit canteens as both timers and critical measurements

Max Scattergun, burst, then continuous
10800

Max Scattergun, 3 bursts
15120

Min Scattergun, burst, then continuous
2520

Min Scattergun, 3 bursts
3780

Max Shortstop
13824

Min Shortstop
3024

Max FaN
11640

Min FaN
3104

Max Popper
11562

Min Popper
3240

Max Bison, burst, then continuous
8638

Max Bison, 3 bursts
13638

Maxed Damage, Clip Size, Ammo Capacity and Reload Speed Beggar's Bazooka

Burst, then continuous
17280

3 bursts
24300

Continuous
13500

Max Phlog
7880
This number is not very helpful on its own because it discounts that the Phlog has infinite crits with no canteen required.
To give an idea, imagine the likely infeasible scenario of needing 15 seconds to acquire 3 more crit canteens.
In this case, the damage per 15 seconds is actually damage per 30 seconds (realistically it's longer), which means each D/15s number here would have to be divided by 30 to get the DPS of the weapon.
The Phlog's D/15s could then be doubled to 15760 to give a better comparison to the other weapons measured in D/30s.
Even in those terms the Phlog would desolate the other weapons in terms of raw tank DPS, but as I've noted, it usually takes longer than 15 seconds to acquire 3 new crit canteens, which varies depending on proximity to the upgrade shop.
This is another reason eliminating threats takes priority over eliminating a tank - your DPS increases as you get closer to the upgrade shop.
The same principle applies when playing as a Heavy with no dispenser nearby - if you conserve your ammunition by only shooting when enemies are close, the increased damage at close range combined with not needing to stop for ammo as often will increase your DPS over time dramatically.
In any case, if you're doing well and attacking a tank early, the Phlog has a clear advantage, and it isn't until the tank is dangerously and undesirably close to the bomb hole (and thus the upgrade stations) that anything comes remotely close to its DPS versus a tank.

Max Rainblower
Approx. 13508

Min Scottish, 3 bursts
5700
Interval is about 10 seconds, so D/15s is about 2850

Max Scottish, 3 bursts
12786
Interval is about 8.5 seconds, so D/15s is about 7521

Demoman is hard to measure. Glad that's over.

Max Demoknight (Zato)
12480

Spirit > Brass Beast > GRU
4807 > 4669 > 3843

Spirit isn't worth it. Use your Beast and the GRU to navigate to key areas.

Engineer with Crit canteens

Sentry + Shotgun
Approximately 6367

Sentry + Max Melee Speed Wrench (default)
Approximately 10164

Wrangler, infinite ammo in sentry
5391

Without crit canteens (uber used for timing)

Sentry + Shotgun
Approximately 3934

Sentry + Max Melee Speed Wrench (default)
4693

Wrangler, infinite ammo in sentry
3053

Infinite ammo in sentry, no Wrangler or other aid
2330

Wrangler boost is only 30%, not 100% like I'd heard. Rumors are fun, aren't they?

Medic - Syringe (Used Blutsauger, which has default damage)
3600

Medic - Melee (Ubersaw, because why use anything else?)
3120

Medic - Max Melee Speed Ubersaw
5265 (pretty impressive, really)

Max Bargain
7800

Min Bargain
1650

Max Bushwacka
6240

Min Bushwacka
3705

Min Revolver
2780

Max Knife
1269

• Build in the path.
  
• Use buildings in the path along with team mates to stand shoulder to shoulder and make a literal wall.
  Again, this is called body blocking and is one of the key MvM techniques which should be applied liberally.
  
• Use chokepoints in this way to completely block their movement.
  Even just one person alone can solo a group of Giant Scouts if body blocking is done well in a good spot.
  
• Milk and Jarate.
  Pretty self explanatory. Respective classes should be using these anyway.
  
• Death Mark with Fan O'war.
  If you miss and only Milk one Giant Scout, Fan the OTHER one so they die fast enough for their speed to not matter.
  
• Airblast.
  Make sure you do it near the damage sources of your team; specifically the Heavy (if you have one) since Heavies have awesome ramp up.
  Other good options are near a Phlog or sentry.
  
• Stickies.
  If Engineer builds at the corner correctly, Giant Scouts should get stuck for a short amount of time.
  Sometimes they get stuck for much longer, but even if they don't, a blast of sticky stuff all at once at the place where they get caught will mutilate them.
  
• Ubersaw.
  As a Medic, since you should be body blocking with your team anyway, take the opportunity to whack the Giant Scout if it's stuck on you for a free uber.
  
• No scopes.
  You probably won't be able to headshot the Giant Scout.
  No scope them in this case.
  
• Explosive Headshots.
  If you DO have a shot lined up and have time to switch to Jarate if you miss, take the shot.
  Explosive Headshots slow targets down temporarily and this can be chained for brutal effect.
  I've triple headshot a Giant Scout to death solo before and it's glorious.
  If the Giant Scout is already slowed by some other means then you should shoot its head anyway for more damage, as well as to keep it slow.
  Don't give "my weapon can't headshot/Explosive Headshot" as an excuse either because if you're using the Sleeper or Huntsman you're Doing It Wrong.
  As a bonus, if you get an Explosive Headshot on a target at the right time, you can catch a Giant Scout or two as they try to slip by.
  Then you can really mess them up during the window of their slowed progression.
  
• Sapper.
  Use Ammo Refill canteens if you need to sap them in quick succession.
  Spy's backstab does 750 at the max upgrade level. This is more than half of a Giant Scout's HP (about 1200 I think).
  Use it.

Largely you should ignore them. If you have a Medic on your team, slowing them down so the Medic can Ubersaw them for a free uber is expected of you.
Beyond that, messing with the Buster messes with the Engineer's pacing and can get him killed or all kinds of other bad things.
Help your Medic out, but otherwise leave it to the Engineer, who should lure it away and then replace their sentry ASAP.

Spy is the best for them.
Sniper is close behind when using the Bargain.
After that you'll probably just want a Demoman or Heavy to crit them to pieces.
Make sure it's at point blank in the Heavy's case - having a Medic helps.
Heavy or Engineer will pop the Giant Medic if they try to snipe them.
Engineer should Wrangle, Heavy should point blank, both should stop shooting if the Giant Medic's health is getting low.
If the Heavy is right up close and still has crits, however, they can usually finish the job.

• Heavy's Knockback makes things icredibly easy compared to other strategies.
  Cover them so they can do it safely; there will be Spies.
  It interferes with their damage, which can be annoying when giving them Kritz, but holding the boss steady against a wall is worth it.
  The fight might take a while but it's like resetting the bomb; would you rather end the round early due to losing or later due to winning?
  Actually, the fight is usually shorter since holding the boss in place allows allies to do damage more efficiently, not less.
  Keep in mind that raging the boss back into their spawn is horrible as it gives them uber again and allows them to heal loads of damage.
  
• Body block. Use Uber canteens if you must (and you do, so use them).
  
• Spy has awesome DPS. You can win without Knockback but it's extremely tedious.
  Damage from stabs is probably the highest DPS available versus the bosses
  Remember to use Uber canteens to safely stab as much as possible without dying to the counter attack
  Try to wait until the boss is distracted by more than one ally so the odds of them turning around are lower
  If the boss is being Raged at the bomb hole they'll rapidly toggle between walking and planting animations, allowing many easy stabs
  Spyblocking is incredibly effective in areas where the boss can't jump over
  Sapper slows them down well, but focusing on stabbing is more important
  
• During Disintegration's boss, spawnkill the Engineer bots or pay for it later.
  
• Bonk! Atomic Punch does not work.
  
• Airblast supposedly doesn't work well.
  The bosses seem to consistently have 90% or so resistance to "vertical airblasts" - the wiki specifically says "vertical".
  This caught my eye, because the strongest knockback I've seen against a Giant of any sort was when a Heavy used Knockback Rage on a Giant Heavy that was being airblasted.
  The airblast picked the Giant up off of the ground enough for the Knockback Rage to have no resistance due to friction, multiplying its effect considerably and tossing the Giant around like a lightweight ragdoll.
  If a Pyro were to try hard enough, perhaps they could airblast a Mecha Engine boss enough for a Heavy's Knockback Rage to be similarly powerful, but I've yet to see this confirmed.
  
• Phlog is awesome. It should be with you anyway if you're on Bigrock where it's the only sane way to handle the tanks there.
  Not so much for Disintegration; airblast is preferred there
  
• I'm told Demoknight doesn't exceed the usual 390 per hit when attacking Captain Punch, who uses the Fists of Steel.
  I guess melee just isn't worth it, so once again Demoknight fails.
  
• Gate closes in beginning area of Bigrock. Don't try to push the boss off of the map.
  Feel free to push them into pits though.
  
• Try to put your sentry in front of the boss while it's distracted and have your allies keep it as such.
  You might even try Wrangling and holding your fire to keep the sentry in the path as long as possible.
  Let that guy just stand there.
  
• Kritzkrieg would be nasty against these guys in theory.
  Most times I play Medic I have loads of money by the last wave and would be able to provide permanent crits to my ally under usual circumstances.
  
• Sniper is awesome for Mecha Engine missions but horrible for their bosses.
  Suck it up and let them play anyway.
  Switching to Phlog might be worth it if enough money is left over for Damage upgrades.
  I once had so much money (A rank thanks to Scout) I was able to switch to Kritz Medic after sniping for all the other waves.
  It was incredibly effective.
  A spare Rage Heavy could be effective too.
  If you stay Sniper, it can be more effective to no scope the boss if you can't safely get close (such as not having Uber canteens left for Captain Punch).
  If the boss is moving in a relatively straight line or is otherwise easier to target (that is, not twitching around due to being Raged), headshotting them can be more effective.
  However, it may work best to use the Bushwacka's crit-on-mini-crit ability to get in close and stop the boss with your body while hitting for crit damage, particularly against Captain Punch (since he takes 390 per hit).
  Note: Huntsman taunt doesn't work anymore. Don't bother.

To the chagrin of more experienced players, Bonk Scouts are often underestimated.
Just like bots that have uber, Bonk Scouts can carry the bomb while invincible, effectively making them Scouts with Pocket Uber Medics.
They should be dealt with the same way Uber Medics and their patients would be dealt with when the are Bonking.
That is, weapons with knockback like explosive weapons or airblasting should be used to keep them back until their Bonk wears off.
Reset them into a hole when possible.
However, since they are "dodging perfectly", as is the flavour of their ability, weapons like the FaN probably won't affect them.
It's interesting to note, however, that unlike bots with uber, Bonk Scouts can be hit with the Ubersaw for free uber - rather safely, since they can't counter attack.

Upgrades can only be sold back during the time before wave 1 starts.

Once you beat a wave you're stuck with your expenditures.

Because of this, changing classes will effectively nullify large amounts of money you've spent and your utility to the team will dissipate along with it.

There are times when it is okay to temporarily change class.

During those times it is advisable to spend little if any money on that class so your main class can benefit as much as possible when you inevitably switch back to it.

The exception of course is if it's actually ideal to make a class change for the current wave, but this is an extremely rare circumstance and should not be planned for.

Examples would be a Demoman for killing Uber Medics, or a Heavy switching later since Heavy's expenditures are largely in canteens in most optimal settings anyway.

A blank Heavy for example can go through many waves while being an integral part of the team without spending ANY money...even on Expert.

All that said, a team should plan to pick their class and their loadout for it on an individual basis with expectations of staying that way from start to finish.

Because of how unforgiving MvM is about making poor choices with class composition of the team, loadout and even the upgrades purchased, planning is easily the most crucial part of MvM.

Win the battle before it starts just as Sun Tzu would by coordinating a general strategy with the team and investing in it exclusively.

If you actually stick to the plan and make no compromises along the way and it's apparent that you're not getting anywhere, you should restart the mission and evaluate what went wrong.

Playing Bigrock without a Phlog is begging for trouble right out of the gate, and there are similar circumstances in other missions to be wary of.

I'm not saying you can't beat Bigrock without a Phlog on your team, but the difficulty difference between having one and not having one is the most extreme example I can think of in all of MvM.

Oddly enough the next most extreme example I can think of is playing Bigrock without a strong spawnkilling class (Demoman or Sniper).

It's much more doable, but still very inconvenient. Learn these sorts of map specific oddities and consider them when joining a team last while inspecting the classes they've chosen while waiting for you to arrive.

Following are what some of the objectives of an MvM game are aside from the obvious one of stopping the bomb.
When choosing a team composition, the idea is to select classes that specialize at each of these objectives such that each one is covered.
They are listed from most important to least.

• Money collection. Without dosh, you can't scale your abilities at a pace competent for the increasing wave difficulty.
  More specifically, operations like using Uber canteens to rebuild uber quickly as a Medic, buy key upgrades as a Sniper or afford Crit canteens for dealing high damage in crisis situations are difficult without funding.
  
• Tank killing.
  You can reset the bomb all day, but you can't airblast a tank.
  Bring some high DPS like the Phlogistinator to melt tanks out of sight and out of mind.
  
• Holding the front line itself is a key objective. That first corner after the bot's spawn needs to be your base of operation for as long as possible.
  If you do lose the front line, it shouldn't be until after the major threats are dealt with, such as Giant Scouts, Uber Medics and anything else that can't be handled in a pinch at the bomb hole.
  If the front line is lost, it should be regained as soon as possible by moving up during a gap in the robots.
  Another benefit of holding the front line is keeping Giant Scouts at bay.
  
• Killing Uber Medics. If there are Uber Medics, they need to be killed before they ever "pop" to achieve best results.
  If too many Uber Medics pop and/or one or more of them pop more than once, the bomb will be driven to the hole with little resistance.
  There's no excuse for any Medic to pop more than once. If one pops, camp it until it wears off and finish the job.
  If the Medic killing fails, pop early in time to finish the job cleanly. Don't lose the front line if you can avoid it.
  Also note that airblasting or otherwise separating Uber Medics from their heal targets is not reliable and is not listed as an objective.
  They need to be killed, not simply stalled.
  That said, body blocking a Giant Medic should be done in addition to other techniques for handling them (backstabs are most effective, then sniping and finally crit stickies).
  Using Uber to stop the Giant Medic and hold it apart from its heal target allows the team to focus fire without accidentally doing enough damage to the Giant Heavy to trigger the uber.
  The extra focus is also good for ensuring that damage is dealt quickly enough to avoid popping the Giant Medic at its sweet spot of around 75-100 HP.
  Finally, while airblasting an Uber Medic away is generally a waste of time, airblasting or otherwise knocking back their heal target back from the team's station is extremely helpful.
  It doesn't matter how many times they pop if they never get past the front line.
  
• Spawnkilling. In general this is necessary to win efficiently and sometimes to win at all.
  The idea of holding the front line is to deal with threats immediately.
  When enemies are allowed to live longer than a few seconds, they spread out, begin to counter attack and ultimately the versatility of their greater numbers comes into play.
  Sniper bots all grouped at the back can be taken out in one shot, Demomen can detonate large groups of dangerous Soldier bots before they even fire and Engineer bots can be killed before they set a single building.
  If they progress more than a few steps, however, they will spread out and they will obliterate you.
  Spy bots often spawn close together as well and just like with spawnkilling other bots, it's either all or nothing - you blast them all ASAP or you ignore them and they disperse, becoming a nightmare.
  Sniper bots are especially bad about this. Sniper bots should be dealt with via counter sniping, Wrangler or any other available means immediately if they aren't spawn killed.
  Sniper bots will spread to optimal locations and with recent updates, their AI and abilities are extremely broken and unfairly overpowered.
  For example, I am frequently hit by 150 damage shots by Sniper bots running at full speed that can't possibly have their weapons charged, but they no scope me while running and kill me in one shot.
  You WILL be pinned in the spawn and forced to "uber out of the spawn" just to keep playing if they are allowed to set up.
  Deflector Heavies, especially the groups of many small ones, can easily godstomp and entire team. They move fast, they deal extra damage, they seem to have reduced fall off and miniguns can shoot through walls (fun fact: this can be used to the player's advantage as well).
  Worst of all, Deflector Heavies have some of the nastiest AI in the game, and the smarter bots in general will actually backtrack to hunt you down so you can't recover and quash them later.
  Unchecked Soldier bots will juggle you with unfair auto aim, firing speed and bottomless clips. Even Uber canteens are useless against them as they will just auto aim you into the air and wait it out.
  Sniper bots and Soldier bots alike will kill you before you can Uber again even if you are mashing your canteen key by exploiting the game's latency with their ridiculously overpowered auto aim and timing.
  Just spawnkill them. Demoman and moreso Sniper are good classes for this.
  
• Bomb Resets. When all else fails, just blow the bomb off the map, or at least safely away from the hole.
  Juggling Giants near the hole is better than letting them cap.
  Airblasting is key for this but not the only tool available.
  Bomb resetting is a last resort for recovering from mistakes, however.
  It is not as necessary of a job as the others.
  If you have a godly Pyro (like me) who can reset the bomb 15 times (yes) in one wave, but your Pyro then has no money for the next wave after carrying the team, you need to restart the mission.
  Just because the Pyro CAN carry you doesn't mean it's okay, and they won't be able to do it forever regardless.
  If they're resetting that much, the rest of you are doing something wrong and need to restart and rethink your strategy.

An ideal team has a class composition that can handle all of the above objectives with specialization.
By specialization, I mean that a class that has the job of "Killing Uber Medics" should be a class that is actually good for doing so.
You can equip the FaN as a Scout to help with resetting the bomb, but you are not the team's designated bomb resetter.
The exception of course would be if you didn't have a dedicated bomb resetter, which is viable but risky (at least, if you're playing with a team you aren't familiar with).
With that all said, here are what the various classes bring to the table when played optimally in the sense of current MvM knowledge:


***** - Godlike
**** - Broken
*** - Professional
** - Specialty
* - Affinity
+ - Boon
: - Perk
= - Neutral
? - No idea
- - Negative attribute
-- - Crippling attribute; picking the class at times when this attribute is relevant is asking to lose
--- - Don't pick Spy. You'll see why at the end.
(I'm mostly kidding; Spy is a fun and effective class that can be used to win missions even when a class like Soldier would be a poor enough choice to cause the team to lose...but it won't be easy for the Spy)


Airblast may be abbreviated as AB.

Pen refers to Penetration, such as for projectiles, or in the case of Spy, armour penetration.

Slow down refers to slow effects from Mad Milk or Jarate.

Handling Snipers refers to being able to pick them off quickly from LONG distances, as opposed to any distance.
Alternatively it may refer to being able to get to them quickly enough to not need range.
Survivability against them is factored in as well, and so are the benefits gained from killing them with a certain ability (specifically, the Ubersaw's uber charging ability).

Uber Medic killing refers to dealing burst damage to kill them instantly so they CAN'T POP.
Obviously any class can kill Uber Medics; it's doing so with the necessary 150 (usually) damage burst that is important.

DPS refers to Damage Per Second, which is related to Damage Per Metre.
DPS ratings below will account for DPS resulting from buffs shared with allies and debuffs applied to enemies.

Corner camping doesn't just refer to hiding around a corner, but doing so effectively enough to cause groups of bots to completely stop at that corner due to being killed quickly when they come around it.
Think of it as creating a black hole of pure damage by abusing your terrain advantage.
Generally, if a class is good at killing bots that come around a corner just as fast as those bots can come around it in the first place, then they are good at corner camping.

Ambushing refers to doing damage to a target from LONG distances that is consistent while retreating behind cover in between shots to improve personal defense at no cost to damage output.
A Soldier can peek around a corner, fire a rocket at a target any distance away, hit it with moderate damage and retreat behind the corner again all in a very short time.
Ambushing thus refers mostly to mobility, although a Pyro's only long distance weapon that works similarly can only hit one target at a time, so Pyro is not good for ambushing.

Attack range refers to the range of the primary DPS utility of the class.
Pyro has plenty of range (their rating for handling Snipers is one of the higher ones) but its DPS is only respectable up close, so it has "no range".

Juggling refers to things like Soldier bots abusing their auto aim and rapid fire to push you into the air and hold you there.
This can also be done by sentries and Heavies; enemies that can juggle are the absolute worst to deal with.
If this attribute is mentioned positively, it's about the player juggling the bots; otherwise it's about the player being juggled.

Building protection refers to being able to break sappers or prevent them from being placed to begin with.

Spawnkilling refers to the class's ability to wipe out large groups of bots as soon as they lose their spawn uber.

Provocation in the context of the Spy class refers to things like
Bots seeing you undisguised
Bots being sapped by you
Bots having their movement blocked by you, which can result from you placing yourself in front of them, or more likely since the recent unfair AI updates, them purposely attempting to walk through you so they can blame you for something you didn't mean to do (this is an effective Spychecking technique in the main game, just as the rapid tiny left and right movements enemy Sniper bots make when you zoom in on them is an effective way to dodge your sniper fire - the bot AI is largely being based off of how actual players play and it is just NASTY)
Bots being attacked by you
Bots being attacked by team mates on the other side of you
Team mates being only kind of behind you such that the bots can attack you by pretending to attack them (they really aren't, they're actually attacking you in an unfair way disguised as a fair way because Valve)
Being alone; if you're alone in an area, or just completely alone on the server, the bots will swarm around you and kill you. If you're cloaked and off in a corner they will actually attack you sooner than if you were disguised. If you jetpack around on a jetpack server you will notice all of the bots underneath you in a crowd. This is ridiculous and honestly I am disappointed that Valve thought this was fair.

Godstomping refers to utter EMBARRASSMENT of the target party. We're talking wave 666 with POOR class choices bomb is taken STRAIGHT to the hole do NOT pass go do NOT collect 200$ HUMILITY.
The bots can be godstomped too, such as when none of them are allowed to take more than maybe 5 steps.
The latter usual entails having a Sniper and some Pyros on your team.

***** Grabs dosh well. Absolute best for it because of extended grab range and healing on money collection.
**** Great healing if Milk is active often enough.
*** Reliable slow down for Giant Scouts.
Recharge rate is more important to a Scout than a Sniper so their Milk's slow is more likely to be used against Giant Scouts than Jarate.
** Great DPS if Fan is used liberally.
** Can lay the hurt down on Mecha engine bosses if they can afford the upgrades.
They probably can because Mecha Engine missions give lots of money.
* Can handle Snipers.
* Can spawnkill Engineer bots.
* Poor damage damage to tanks normally, but if they can afford the upgrades they're okay for it.
+ Auxiliary bomb resetting.
+ Auxiliary Uber Medic killer.
+ Can survive Spies.
= No body blocking traits.
- No range.
- Awful for spawnkilling.
- Building protection? When is a Scout near a building? When they're ignoring dosh and hurting the team, that's when.
If a Scout DOES need to stop for a bit of dispenser love they still aren't going to be any help against a swarm of Spy bots.

(Buff OR Bison - many points apply to both and those that don't will be noted)

**** Tank buster (Buff Banner build with Beggar's).
Would be rated higher, but because the Buff Banner requires allies to be near you, it prevents them from multitasking as well as they otherwise could.
Also note that the extremely high DPS numbers in the tank section ignore the fact that most runs will never have ALL of the upgrades used for the Beggar's Bazooka's test run.
In addition, all of the numbers in that section are from having permanent crits, which is only possible if the team coordinates to bust the tank in 15 seconds (doable, but rare).
**** Tank buster (Bison).
Would be rated lower, but can afford extra Crit canteens due to the Bison maxing early.
**** Can dispatch Engineers even at range.
*** Good range.
*** Buff Banner gives good DPS to entire team fairly often.
* Can handle Snipers.
* Can ambush.
* Can do serious damage to Mecha Engine bosses when coupled with Buff Banner.
* Decent spawnkilling.
+ Auxiliary bomb resetting.
+ Auxiliary Uber Medic killer (Buff build only; requires crockets).
+ Can survive Spies.
+ Can move around quickly without a teleporter.
+ May be able to blow Spy bots away, but them doing so before the Spies place any sappers is less likely than with other classes.
: Can burst damage Giant Scouts (Buff build only; requires crockets).
= No body blocking traits.
? No idea how their healing is; I only ever see Buff Banner used for buffing.
- Bad for grabbing money, although they can rocket jump.
-- Garbage in missions with lots of Pyro bots and/or Deflector Heavies (Buff build only;
Bison is immune to this problem whereas the primaries can have the added bonus of
killing your team mates...when this happens it is YOUR fault for firing at a
dangerous target, not theirs for engaging it while you were being a jerk).

***** Bomb resetting.
***** Giant Scout embarrasser.
***** Juggling in general (better than body blocking).
***** Building protection.
Best because they can also airblast uber enemies off of buildings.
***** Spy killer. Best because they can also airblast Spies away if they can't kill them in time to save someone otherwise.
*** Tank buster.
*** Can handle Snipers well.
** Strong corner camping.
** Good for grabbing money since most of what they kill dies near enough to them to make grabbing the money easy.
* Reflection.
+ Auxiliary Uber Medic killer (3rd Deg, although sapper killing is preferred)
+ Does okay damage to Mecha Engine bosses.
: Decent DPS.
: Can spawnkill sometimes.
= No healing.
- No range.
- Can't ambush.
- Juggled easily.
- Garbage for Engineer bots.

***** Tank buster.
**** Ridiculous DPS.
Barely outclassed by Sniper and only because of how many more bots there are than tanks.
Absolute best class for taking on waves with low player counts.
**** Spy killer.
Remember to taunt in corners and circle tanks instead of leaving yourself open to backstabs.
**** Building protection.
**** Corner camping with Phlog? yesplz
**** Can burn up Mecha Engine bosses roasty toasty like whoasty.
** Good for grabbing money since most of what they kill dies near enough to them to make grabbing the money easy.
** Can handle Snipers.
** Auxiliary bomb resetting.
** Full heal on Phlog taunt.
+ Auxiliary Uber Medic killer (3rd Deg, although sapper killing is preferred)
: Can spawnkill sometimes.
= No body blocking traits.
- No range.
- Can't ambush.
- Juggled easily.
- Poor with Giant Scouts.
- Garbage for Engineer bots.

***** Best ambush.
Sniper is tied because of greater DPS, but Demomen using stickies are incredibly safe if they don't stick their heads out like an idiot.
**** Uber Medic killer.
Broken rating because even a blank (that is, no upgrades) Demoman is often enough to completely handle the job of killing Uber Medics.
Not godlike, though, because Demomen have so many things that can go wrong (like Spies or bots avoiding stickies, which they do egregiously).
**** Juggleproof. They're always behind a corner so why would they ever be juggled?
**** Giant Scout obliterator.
*** Tank buster.
** Spawnkilling.
** Strong corner camping. Would be best, but does not have continuous fire.
** Powerful solution to Engineer bots' nests.
** Excellent DPS.
** Moderate range.
** Auxiliary bomb resetting. Improved thank to Loose Cannon.
+ Okay damage to Mecha Engine bosses.
+ Might not be able to prevent sapping, but will surely punish any Spies that get near a sticky trapped building.
= No healing.
- Positively eaten alive by Snipers.
- Completely snack food to Spies.
- Poor at body blocking since counter attacking while blocking will hurt themselves (they should be using stickies).
- Horrible for gathering money.

**** Remember how you're not supposed to melee the tank? Well, the Demoknight is one of the bigger exceptions.
Demoknight with high Damage and Melee Speed is actually a better Kritzkrieg target than many other classes.
** HP On Kill all day.
** Can defend against Spies decently enough.
** Can deal solid damage while body blocking.
** Can gather money well since they have to be right on top of everything they kill anyway.
: If they body block a Giant Scout, they'll be able to melee it to pieces.
: Might be able to use a charge attack to push a bomb carrier off of a cliff. Probably not, though.
- Can't spawnkill for beans. Can't even hit more than one target at a time to begin with.
- Can't corner camp. Range is too short, even for something that requires as little range as corner camping does.
- Useless against Engineer bots.
- Can't guard a building for beans.
-- Poor damage to Mecha Engine Bosses due to always being in danger.
Utterly useless against Captain Punch.
-- NO ambush. At all.
-- Horrible for killing Uber Medics.
They need crits to do so effectively.
Either they need a Crit canteen, in which case they will not have an Uber canteen and will likely get dumped on, or they need to kill another enemy nearby so they can have Crits On Kill active, which is hard to do reliably even if their team mates aren't interfering (which they will be).
Beyond that, their only option is to crit the first Medic they charge.
Even if they succeed, at higher difficulties they may either be shredded shortly after or when their Uber canteen wears off.
Ultimately their inability to target more than one enemy at a time and their weakness to being juggled means that relying on them to take out Uber Medics is folly.
A good Demoknight can surely do it with some reliability, but at that point most of this guide is inapplicable anyway (this guide is to help minimize skill requirements and difficulty, not test them).
-- Jugglefood. Jugglers eat Demoknights for breakfast, lunch, dinner and then again after pooping them.
Demoknights typically have higher resistances than others to explosions, making the primary source of juggled players less damaging to them.
However, like with Sniper, Demoknights are completely incapacitated and unable to counter attack when juggled.
Unlike Sniper, though, Demoknights have garbage range and can't recover. They're just plain irredeemable.
-- No range.
-- NO DPS. Can only target one enemy at a time and is always in danger when doing so.
-- Utter Sniper food.

Demoknight is so terrible. I won't bother adding weird builds like "Flare Pyro" or "Shotgun Heavy" because it's just obvious they're ineffective.

***** Divine corner camping.
***** Knockback Rage can push back Mecha Engine bosses.
***** Body blocking pros.
Engineer buildings are less mobile, but only the Heavy has the insane DPS at point blank and the resistance to knockback to body block like a nonchalant god.
A Heavy with resistances or an Uber canteen (sometimes a Crit canteen if they have some means to survive) can walk right up to a Giant, stop its movement and kill it where it stands.
Incidentally, not enough players take advantage of this and lose DPS by standing too far away and/or letting the bots walk on by.
Treat a Giant like you would a tank if you can afford to and just plain body block it.
If nothing else you'll look cool.
**** Juggle resistance.
*** Stupid high damage at close range.
Projectile Penetration helps with DPS.
* Knocback Rage is pretty much the only way to reset a giant beyond airblasting.
* Good at killing Giant Scouts if they hold onto Crit Canteens, stand in the path and dump crits into them.
Heavies are usually responsible for killing Giant Scouts after help from their team mates is applied.
They're actually horrible for it in general because they (obviously) can't catch them - use Crit canteens if necessary and use your girth to hold them in place.
* Moderate range. They have poor long range due to fall off and excellent close range due to ramp up.
At moderate range they're about even with a sentry, and they can use Crit canteens liberally to reduce fall off and give themselves incredible range.
Kritz Medics give them good range as well.
* Usually on a dispenser and will therefore be able to protect it fairly well if they Spycheck like they should.
Still, something's bound to be sapped if the Heavy is defending it.
+ Can share Sandviches.
- Can't ambush. Doesn't need to anyway.
- Can't spawnkill for beans.
- Poor damage to tanks.
- Sniper food.
- Spy food.
- Horrible for killing Uber Medics.
Usually responsible for popping them instead. Learn to let go of the trigger, geez.
- Horrible for handling Engineer bots. The time it takes to slog through their sentry fire to get close enough to be effective isn't worth it even if the resources are available.
- Bad for grabbing money, although with GRU they can occasionally make a worthwhile trip to grab dosh.

***** Dispenser healing is top notch - it also gives infinite ammo.
***** Perfect for dealing with enemy Snipers - they have range and a shield to hide behind while counter sniping.
***** Juggleproof. Buildings can't be juggled, silly.
**** Powerful body blocking. If you want a Giant Scout stuck standing around like a fool, a sentry stuck in a chokepoint is a good way to say "lolno" to Giant Scouts.
**** Obviously good at defending their buildings. They're alerted when it happens, which one it is, should have Melee Speed to help and above all, they can repair the damage done by a Sapper...
But only the Pyro class is reliable enough to prevent sapping entirely as well as break sappers with the pure speed of the Homewrecker to completely minimize damage and duration of incapacitation.
**** Good at dealing with Giant Scouts because of body blocking ability.
**** Excellent range.
*** Reliable corner camping.
* Effective at stopping Mecha Engine bosses with cunning sentry placement.
* Can Wrangle enemy Engineers effectively, although many times the enemy Engineers will set up out of view.
In these cases the Engineer can't really go after them without sacrificing much of their utility by undoing their building placement.
In that case, it's better to let allies handle it.
* Not completely terrible at dealing with Spies. Sure, they will target buildings with high priority, but:
Spies are more likely to do what other support bots do and target MVPs first. These will be things like the Sniper, the Medic, the Demoman or the Heavy (in that order, assuming they're good players).
Engineer will usually invest in Melee Speed and be able to just knock the Spies around.
Engineer's buildings and the Engineer themselves are usually near allies that will help.
Wrangler.
- Some of the worst DPS in the game.
Their supposed DPS comes from the sentry's auto aim stealing kills with inhuman accuracy, but actual analysis in conditions in which aim is not as important (and generally it isn't) sentries have garbage DPS. Even when Wrangled. This is less true during earlier waves when some classes aren't as overpowered yet; don't afraid to temporarily go Engineer (though if you have one already it's better go temp Heavy if it's for an early wave; if it's for a later wave going temp anything is usually suicide).
- Can't reset the bomb like, at all.
- Can't ambush, doesn't need to.
- Horrible for spawnkilling.
- Poor damage to tanks.
- Horrible for killing Uber Medics.
Like with Heavies, they are usually responsible for unwanted pops.
Again, learn to hold your fire or redirect it somewhere less terrible.
- Grabbing money is just not something an Engineer should be doing.

**** A good Kritzkrieg Medic has DPS comparable to a Phyro or Sniper.
No, seriously. This is one of the most broken set ups in the game.
**** Medics themselves are poor for corner camping, but they greatly magnify its effect for their heal targets.
With the Kritzkrieg, a Medic can give a corner camping Heavy such incredible power that it's often enough to trash a Giant in just the time it takes them to turn the corner.
And that's without any fancy upgrades, or at least if some defense is needed a simple Uber canteen is plenty enough.
**** Can build ubers while body blocking.
Should be using Uber canteens to uber safely off of dangerous targets anyway, meaning they will be body blocking more than most players.
**** Because a Medic can body block well with Uber canteens to both stop enemy movement and quickly build ubers, a Medic is excellent for trashing Mecha Engine bosses.
**** Medics can trash enemy Snipers by using their Movement Speed to close the gap and build uber in the process.
**** Medics themselves do not have good range, but with the Kritzkrieg they can reduce fall off and improve damage of allies.
In effect, a Medic can make a Heavy into a sniper and aid allies in obliterating distant targets.
**** A skilled Medic can lure Spy bots into front stabbing to lower their disguise, then build uber off of them while they are undisguised.
**** Powerful ally against tanks.
If you don't steal their kills and help them with Busters, they can even get loads of extra free ubers to share for the tanks.
A Pybro's best friend during tank segments.
*** Excellent healing, but Medic is not best used for healing (derp).
Medic also does not give ammunition like a dispenser does (sharing Ammo Refill canteens is usually a waste).
In addition, Mad Milk healing scales with DPS - if you really want to heal someone, give them a Kritz uber versus a Milk'd Giant.
That said, a Medic can safely build uber by healing when they're unable to Ubersaw to get that last bit of charge for the next crucial uber.
*** Medics help with Giant Scouts by body blocking with the others and building free uber in the process.
*** Similar to how a Medic can buff allies' corner camping ability, so can a Medic improve the ambushing techniques of, say, a Soldier.
This is really only relevant to the Kritzkrieg.
** Medics don't defend buildings too well, but if they're on a Heavy they're usually near them.
Because they can sometimes get a lot of their uber charged up by hunting Spies, having a Medic defending buildings is particularly effective.
* A Medic can share uber, or if they're being silly and using the Medi Gun, pop one, to help an ally destroy an enemy Engineer's nest.
* Medics are actually good for grabbing dosh:
They move fast
They're usually uber if they're out in the field while enemies are about, or else it's safe enough that they don't need to be
If they're using a Medi Gun they can grab dosh while running in a circle around their heal target
This all assumes the Medic is any good, of course.
* No bomb resetting tools, but Medic can pop or share Uber with a Pybro to really get work done.
* Can help with spawnkilling by giving spare Kritz uber to a Demo.
Heavies are poor for spawnkilling, but with a Kritz Medic they can actually do it well.
This is another reason why a Heavy should carry Crit canteens all the time if they have no Kritz Medic (and sometimes even if they have one) in missions where survivability is possible with raw DPS (i.e. most if not all of them)
* A Kritz Medic can help a Demoman take advantage of a break in the wave to prepare crit stickies for upcoming Uber Medics.
- Juggling hurts Ubersaw charges.
However, it can be used to the Medic's advantage to rocket jump back to their heal target before their uber wears off, potentially saving an uber from being wasted.

***** Absolute highest DPS in the game.
Explosive Headshots are just overpowered, especially since the Expert mode update.
A simple test on my server showed that the groups of 4 Gauntlet Heavies crowding a Giant Demoman at the beginning of Mannslaughter can have 1240 damage done to the group per shot with a single Explosive Headshots upgrade.
Applying Jarate to them before firing bumps this up to approximately 1600 per shot and begins to outright kill the Gauntlet Heavies after 2 shots.
Even a solo Giant is torn apart by headshots, which is something the Sleeper can't do.
Huntsman doesn't even have Explosive Headshots, so it's actually worse.
The highest DPS is generally from the Bargain because of its reliability and then the Heatmaker because of its focus.
Other rifles just don't cut it.
Sniper is actually about in the middle of the class list for effectiveness against tanks and trashes everything else.
If a good Sniper is on your team, don't expect to do more damage than them. It just won't happen.
***** Sniper class obviously has good range. Sniper has INFINITE range. That's pretty godlike.
***** Because of their range and damage, Sniper is THE best class for spawnkilling.
A Sniper is basically what a Demoman would be if they could fire all of the stickies in their launcher each time they clicked.
Only Sniper still has more range.
Now do you see why Sniper has so many godlike ratings? You're blind if you don't.
I've literally had people say "why did those Soldier bots randomly die?" because they didn't realize they had been caught in an Explosive Headshot.
People actually boss other players around for picking a class they don't like such as Sniper (even though Sniper is good...?) because they don't know what Sniper can do.
If you don't know what the classes can do to some high level of accuracy, you probably shouldn't judge them.
***** Masters of corner camping.
Many walls are see through, such as fences.
Often, a Sniper will see a target go behind one wall and line up the shot for its reappearance.
A good Sniper can retreat behind a corner after firing a headshot and repeat this process in a circular movement around said corner to trash Giants and mobs.
As a bonus, the bomb carrier will be outlined, making it hilariously easy to corner camp them.
***** Masters of ambushing, too. A Sniper can pop out, fire and insanely powerful Explosive Headshot and retreat behind cover with ease.
They can do this rapidly, and a good Sniper *raises hand* can even do this at mid to close range to hold down a corner -or even an open field- until allies return.
***** Ridiculously good for killing Uber Medics.
There are many times when an Explosive Headshot will pop one rather than killing it, but if the Sniper is good, they will cause this early on.
With the remaining Uber Medics popped insanely early and most of their HP missing, they're just about guaranteed to never pop again.
Combine that with the fact that just as often an Explosive Headshot will wipe out every Medic in range, from 1 Medic to 10 of them (highest I've seen is probably about 9; not sure), and you have your Medic Killer right here.
Why pick Demo if you can have the DPS of Sniper?
*** If a Sniper can aim at an Engineer bot's nest from outside of the sentry's range, that nest is toast.
Snipers are completely beast against Engineer bots, especially at Disintegration where they're usually in view from just about anywhere.
What's more, if the Sniper gets enough Damage and Reload Speed upgrades, they can deal damage faster than the Engineer bot can repair it.
This way they can rapidly no scope the sentry to bits in the event the Engineer bot is hiding behind the sentry to avoid being hit by a headshot.
** As was said, a properly upgraded Sniper is actually at about the middle of the list of classes that are good against tanks.
A Sniper can do more damage to tanks than a Heavy or Engineer with fully upgraded Damage, Reload Speed and Crit canteens.
Perhaps just as important, a Sniper can use their incredible power against pretty much every other threat to keep distractions off of allies so they can focus on the tank(s).
* Explosive Headshots can slow Giant Scouts, but more importantly, Jarate can apply Slow on them.
However, because a good Sniper will want to invest in upgrades that are better overall, they will probably not have the Recharge Rate on their Jarate to help with Giant Scouts as much as other team members.
* Can effectively no scope and even snipe enemy Sniper bots sometimes, but because of their ridiculous DPS (see above),
Sniper bots will target good Snipers with unrelenting prejudice.
The result is that the low HP of the Sniper class will require some combination of:
Bullet Resistance
Help from allies
Luck and skill with positioning against them
Otherwise, the Sniper player will be eaten alive by the enemy Sniper bot(s).
+ Snipers can handle Spies moderately well with Jarate + Bushwacka, but because Snipers are often
Alone
Priority targets due to their Uber Medic killing and damage prowess
Not Pyros (being a Pyro is a good way of making yourself unattractive to Spies)
The Spy bots will often SWARM a Sniper (especially a good one). Because of this, Sniper isn't quite "good" for dealing with Spies, but I've handled 4-8 Spies on my own with just Jarate and Bushwacka, so I know it's doable.
+ If a group of Spies assail a building, a Sniper can Jarate the lot of them to make sure they pay for what they do.
= No healing.
= No body blocking traits.
- Snipers do poor damage to Mecha Engine bosses because of hit detection issues and things the boss and the Sniper's allies will be doing to affect its movement such that clean shots are less viable than they are against most Giant enemies.
Sniper can help with marking by using Jarate, but other classes such as Scout or Soldier will probably help enough to make the Jarate pointless.
- Please do not make your Sniper stop to get the money by leaving it all on the ground in front of them.
Snipers do so much work as it is; don't make them do literally all of it.
- Snipers can't reset the bomb.
If you let them get assassinated and the lack of having them alive costs you the DPS your team needs to hold the front line, then you'd better reset the bomb for them.
Don't punish your Sniper by having them feel like they have to uber out of the spawn with a massive close range headshot spree to save the day.
It's your fault you were a bad team mate to them and lost your strongest player, not theirs for picking the best class.
- Snipers are usually in a good position to avoid being juggled to begin with, but Snipers can have their aim knocked aside even by hitscan weapons.
Because of this, a Sniper can be rendered completely useless by even the slightest disruption and so their rating for dealing with juggling is extremely low.
However, if the team isn't garbage, the other players will do their part to protect the Sniper and the Sniper, if they're any good, will do their part to avoid being shot.
The result will be all of the advantages listed above with this disadvantage made largely irrelevant.
So as a tip to players of other classes: COVER YOUR SNIPER. You will be helping your team so much more by taking the extra few seconds to kill those Spies or stray Scouts than if you focused on doing the damage yourself.
You can't do more damage than a good Sniper anyway unless the team is doing so bad that all of you are getting godstomped.

***** Body blocking gods.
There's a reason the term "Spyblock" was coined.
With the new, extremely unfair and broken bot AI, Spies can no longer Spyblock as effectively.
It can still be done, however.
It used to be you could just stand in a chokepoint and the bots would bunch up against you and become fish in a barrel.
Now, however, bots have learned to jump, and can even simply walk over you without the jump animation.
They even do this to other classes if they're ubered in front of them for too long.
In addition, pretty much EVERYTHING will Spycheck you.
They even do this even without any provocation.
However, there are ways to counter these things.
The jumping and walking over can be countered by blocking for short periods repeatedly.
Wait them out for a bit, then let them move a bit, then wait them out again.
The cycle isn't as effective as the old perfect immobilization method but it's better than nothing.
Some rooms have low ceilings. They plain can't get past you in these rooms so you can Spyblock indefinitely, except for that they will attack you after a while.
To counter THAT, use the Dead Ringer. Stay disguised to buy time, but have the Dead Ringer ready.
Tank a few blows and then recover. A Medic or dispenser would assuredly help.
In any case, if Spyblocking something that isn't a Giant Scout, you are probably doing so to exploit the game and not to win with a normal strategy.
In this case you are going to have to go the extra step and have your team coordinate their fire so that the bots are blocked by you without being provoked into attacking you.
Bots will intentionally attack you if an ally of yours is behind you so the game can "blame" your allies for drawing fire when they're really (and obviously) looking for an excuse to kill you without good reason.
***** Nothing can clean up an Engineer nest like a Spy, just as in normal play.
***** Spies trash enemy Sniper bots.
Even if they have a Razorback, a Spy can reliably and safely shoot them from just outside of the range that makes them melee.
**** Spies can grab money effectively.
They can even do well enough for the team to not need a Scout and still afford the usual high end upgrades.
The fun part is that while they don't get health from grabbing dosh or have extra grabbing range, they DO have the Dead Ringer and their disguises to grab the money safely.
In addition they are of great utility against Giant Scouts and Uber Medics, effectively allowing them to compete with Scout with respect to what the Scout is good at while having an edge in Uber Medic killing.
It is because of this that Spy is not the worst class choice for MvM despite the unfair AI updates that specifically hate on Spies.
**** Spy is excellent for handling Uber Medics.
They can sap a whole group to cleanly deal with them all.
They aren't as effective against Medics that spread out, but even Demoman and Sniper have their difficulties with that.
Most of all, though, Spy is the absolute best class for taking out Giant Medics since they don't turn around when stabbed.
*** Spies actually do excellent damage to Mecha Engine bosses.
If the boss is facing the opposite direction of the way it's moving, the Spy can even body block for a bit while they deal massive damage.
** Sapper with Ammo Refill canteen is excellent for dealing with Giant Scouts.
In addition a Spy can take out huge chunks of their health with Pen, doing more than half their health in damage with a single backstab when fully upgraded.
** Spies don't handle juggling too well, but because of the Dead Ringer it's not really an issue.
The real problem is all the stray fire "at your team mates" (read: at you, disguised as being at your team mates) can meatshot you into oblivion, often interfering with your movement anyway.
* Spies can counter bot Spies with sapper, but it's expensive and wasteful.
However, since Spy bots often walk backward to be able to backstab the moment they finish noclipping through your solid body for the cheatstab, and because they don't facestab you if you're disguised (usually), you can sometimes backstab then just by timing a swing when they're walking backward into you.
Which they WILL do, because ALL bots congregate near you when you are a Spy and try to noclip inside of you to block your movement.
They also use this as an excuse to Spycheck you and they WILL kill you while you're disguised with NO provocation.
* Spy is decent at spawnkilling because of sapping, but it takes more resources (time and/or Ammo Refill canteen) than even the Demoman's stickies (which take quite some time to set up).
* A Spy isn't going to reset the bomb, so you'd better do it for them.
However, a Spy can sap a key target near a place that is easier to reset from to help out their allies.
* A Spy can sometimes sap a group of enemy Spies on their way to an ally's building and handle all of them.
In my experience as a Spy, however, this doesn't happen often because Spies are usually busy doing more important things (like killing Uber Medics).
+ The closest thing to corner camping a Spy can do is to backstab some enemies near a corner and then retreat to avoid wasting their Dead Ringer.
It's extremely effective against Giants but not much else.
= No healing.
- Garbage DPS. My damage record is like 11000. I was backstabbing Giants like crazy and still flopped.
To give some perspective, most classes would have averaged 20000 in the same situation.
I did 11000 playing extremely well, the best Spying I've seen to date.
- Range? What range? Your best weapon is a knife.
- Ambushing is pointless for a Spy. They could use it to take out Sniper bots, but it's not worth shooting anything else.
-- Worst DPS for tanks. So bad that you might wish you had never chosen Spy.
You might even have to "temp Phlog".
--- The Bot AI is extremely broken after recent updates.
Bots walk right up to disguised Spies and kill them without provocation, among other horrible things.
The new bot AI is so bad it even affects other players, resulting in bots merging inside of ubered players to freeze their movment and just generally exhibiting poor game design.

After all of the above, the team that does the best with specialization (which is effectively looking at the list of jobs and picking a class with a 5 star rating for that job for each) would be:
Scout, Pybro, Phyro, Heavy, Engineer, Sniper

And indeed, this team should be capable of breezing through any of the MvM missions avaiable at any difficulty.

Some other team examples can be obtained by tweaking this base team with replacements that exchange one of the classes for another that is similarly good at the job the original class was good for.
Of course, since the base team is optimal, you're effectively hurting your team by making such replacements - in theory.
Obvious factors that would affect whether the above team example is optimal are player skill, money amount for the current wave (Heavy performs better when money is low, Sniper performs better when money is high - the ratings above were overall ratings over the course of the average entire mission) and bot composition (which is obviously going to be rather distinct between missions).

But anyway,
Scout, Phyro, Demoman, Heavy, Heavy, Engineer

This one is extremely popular. Sniper is swapped for Demo and airblast is deemed unnecessary (this is often true for a good team anyway) and is thus swapped for another Heavy.
This set up is extremely powerful in the Mecha Engine missions where Heavy is desired for the bosses.
However, it is often used and abused in Expert missions where it is not as ideal.

The other most popular variant is
Scout, Pybro, Demoman, Heavy, Heavy, Engineer
Same concept, only the Pyro is expected to reset and use Crit canteens on the tanks.
A slightly more defensive build than the previous example and very prominent early on in the history of MvM.

A personal recommendation to keep things interesting is
Scout, Pybro, Heavy, Engineer, Medic, Sniper

Here, Sniper returns to his rightful place as the high DPS Uber Medic killer.
Pybro reigns supreme over the bomb carrier and the Medic does more damage (in effect, anyway) than the second Heavy would have done.
First by giving Kritz to the Heavy, then later to the Pybro for tanks.
Medic can be expensive depending on how smooth things go, so Scout is chosen over Spy for money grabbing.

It's interesting to note that since Pyros are good at grabbing money, Spy is okay for it, and Spy is better at killing Uber Medics, a Spy can actually be better than a Scout in the "ideal team" set up.
They bring in their powerful Spyblocking technique for emergencies, still get money well, and many teams often struggle with Uber Medics when there's only one class for killing them.
Usually, it's enough to have a Pybro airblast the uber enemies back until the uber wears off if they happen to pop.
However, with this team:

Pybro, Phyro, Heavy, Engineer, Sniper, Spy

You can reset the bomb easily, melt tanks, Rage Mecha Engine bosses, provide healing and ammo to the team, slaughter the bots, speedily dispatch enemy Medics with not one but 2 Uber Medic killers, have plenty of money and ultimately still have the option to Spyblock.

If you want to get frisky, you could exchange the Phyro for a Kritz Medic and lose some of the excess tank DPS in exchange for hilarious bot DPS and, more importantly, have a team with every support class that still functions better than most teams.

If your DPS is hilarious anyway, though, you shouldn't need to reset the bomb, and you can probably do it with the Scorch Shot or Knockback Rage in some cases anyway.

So roll Phyro, Heavy, Engineer, Medic, Sniper and Spy and giggle at all the new2mvms playing and losing with dual Heavy + Demo.

I personally like to buy damage to help with instantly killing Uber Medics, but it's not a priority.
With FaN, Scout can handle Uber Medics, resetting and do great damage while collecting money.
Very powerful versatility. Soda Popper and Shortstop have good damage.
Scattergun is more reliable if low on money for upgrades to the primary but is easily outclassed.
Mad Milk all the way. Other secondaries just aren't useful enough for someone who is busy collecting dosh.
Same concept with Fan; you'll be hitting and running since you're busy collecting money, and if you do stop to do damage it will be with your primary anyway.
Milk and Fan. MILK AND FAN.
It's important if there's a (good) Medic on your team to Slow Milk Sentry Busters so that the Medic can Ubersaw them.
Don't stay behind enemies too long when running around to grab money.
Luring enemies backward can cause them to face away from the group, making the job of Snipers and Spies on your team unnecessarily difficult.
Using the FaN's knockback unnecessarily can have similarly undesirable results if you aim at a target your Sniper is trying to target.

My personal upgrade path is Slow, Movement Speed, Milk Recharge Rate, and then a mix of Damage and Clip Size. With FaN, Clip Size yields higher DPS than Damage at first, but at least one Damage is needed to help with Uber Medics (preferably 2).

Uber canteens all the way, but Crit the tanks (and Giants if you can afford to pitch in).

Beggar's is good for some things, Default is good for others.
I've heard good things about the Black Box.
Don't use Direct hit, please. Seriously? Liberty is bad too.
Buff Banner all the way.
Rocket Jumper and Bison for Mannslaughter because of all the Deflectors and Pyros.
Yes, you SHOULD use the Jumper if you're maining the Bison.
You won't be using your primary for damage anyway, so you'll want the extra mobility of jumping without blowing yourself up.
Bison has highest DPS on tanks within 15 seconds (supposedly); it's about even with the Phlog (no seriously, we tested it on my stats server) but is outclassed over time.
Disciplinary is a good team Melee; Escape Plan otherwise.

General upgrade path is probably Reload Speed, HP On Kill, then Damage, but I don't Soldier much anymore. Soldier is outclassed by basically everything.

Crit canteens - Uber's a waste since you can snipe and camp corners/battlements.

Airblast Pyro. Should focus on airblasting.
Probably shouldn't buy resistances since you should be Uber while doing dangerous resets.
Use default; Deg isn't worth it and Backburner is horrible.
Flare Gun for Snipers. You have AB, you don't need to Scorch Shot.
Homewrecker to one hit sappers. Help your Engineer. No Engineer? 3rd Deg + Crit the Uber Medics.
Axt is garbage, quit using it. What's wrong with you?
Please, please PLEASE airblast NEAR the damage source when stopping Giant Scouts.
Another good reason to do this is to avoid interfering with your Sniper's aim, which is a huge waste of damage.
In fact, don't airblast anything until it's near your front line's damage source.
Keep Ubered enemies from getting past the front line, and stop Giant Scouts right in front of your damage dealers so they can safely take them out instead of doing poor damage from the back, or worse, dying when they come up to the front where you stupidly airblasted them too early.
It's important if there's a (good) Medic on your team to hold Sentry Busters steady with airblasting so that the Medic can Ubersaw it.
Don't bother doing this if the Buster is being slowed by Milk or Jarate, however, because it's easier to hit in that case than when it's being airblasted.
It should go without saying that you're better safe than sorry - pop your Uber canteen if there's even a slight chance your bomb reset will be dangerous.

General upgrade path is AB Force, Ammo Cap, Movement Speed, Damage.

Uber for resets, Crit for tanks.

Phlog Pyro.
Second highest damage just after Sniper, highest damage to strong tanks (and strongest damage to everything in general, though Sniper frequently wins due to high bot to tank ratio).
Best class in the game, good at everything.
Bring Scorch Shot to reset - it's also good for setting many enemies on fire at once to safely build Mmmph.
Be sure not to spam it unnecessarily and irritate your Sniper.
Use it to keep uber enemies pushed back if they've popped.
Same melee as Pybro; HW (Annihilator's extra hit requirement is bad; wet targets are gonna be dead anyway) for sappers, 3rd Deg for Uber Medics.
When fighting Giant Medics, place your Uber self between the Medic and its Patient to separate it, hold it in place and deal maximum damage.
When not fighting at a corner, run in tight circles and hit enemies at the edge of your flames to increase your reaction time and minimize your chances of being hit.
You should really be at a corner, though.

General upgrade path is HP On Kill, Damage, Ammo Cap, Movement Speed. Reorder as necessary.

Uber canteens. You shouldn't need Ammo Refill.

Demoknight is a separate class. I don't know a thing about Demoknight except that it sucks, so I won't cover it.
Fast arm time of default Stickies is nice but not worth it. You want the SR.
Either toonker of Loose Cannon and Loch is fine.
4-pill can jump better than LC and both can handle mobs better than the Loch.
Loch is better for handling enemy Snipers.
The LC is pretty much a direct upgrade over the 4-pill, however, because it adds knockback without any real penalty.
That said, be careful you don't knockback targets that, for example, your Sniper might be targeting.
Zato is okay but risky. Risk might be worth it, but I switched to the Sign (default stats).
Problem with Eyelander is that you're not a Demoknight (ew) so its disadvantage is more prominent and the head collecting is nigh non existent (Demo is a "safe" class and should be hiding anyway, not building heads).

General upgrade path is Damage (to avoid accidentally popping Uber Medics instead of outright killing them), Reload Speed, Firing Speed.

Should use Crits. Don't need anything else; just hide and bomb.

This should be obvious, but if you're arriving at the position you'll be defending rather than already being there, you should jump and rev your gun in the air.
This allows you to travel the last bit of distance to your destination with your gun mostly rev'd up already.
This is especially important in keeping the heat on the robots if a Buster stops near you.
Jump out of the way of the Buster instead of simply moving so that you can start firing sooner after it goes off.
Then jump back to the corner you left next time it's clear.
You can also use this technique to ambush robots effectively if you get displaced, which you might do on purpose.
I call it "Batmanning" or "being Batman" when I dive onto a Giant robot's head from a ledge and smoke them.
Sometimes this can be done before they can react, but you can use an Uber canteen to kill them safely with maximum ramp up if necessary.
Particularly effective against Giant Medics.
Speaking of Giant Medics, if you have the Uber canteens for it (which you should), you can body block the Giant Medic by running between it and the Giant Heavy while uber.
From there you will separate the Giant Medic from its patient, allowing the team to focus on it, and do maximum damage due to being at point blank.
Usually, you should only buy Crit canteens and crit everything that moves.
In missions with lots of 100% crit enemies, buy Resistances instead.
Crit first, then (usually) Blast.
Fire first if mostly Pyros are coming; Bullet first if there's nothing to Blast Resist.
If it's just plain not safe to spam Crits, spam Ubers instead.
Deflection is an appropriate upgrade in missions with lots of money, which tend to also be the ones that call for Resistances.
However, Deflection in general is a horrible waste of 600$ that could be better spent on things like a 75$ uber canteen that could last long enough to kill all 3 Giant Demomen huddled together with 525$ left over for more canteens.
Just a single Pen upgrade can complement a Crit canteen run by doubling your damage, but a second upgrade would only multiply damage by 1.5 afterward.
Even less after considering how uncommon it is to have a straight shot through 3 Giants at once. Don't buy more than one Pen, but consider buying at least one.
Please please please move close and point blank enemies when possible.
Heavy's ramp up is godly and his fall off is atrocious.
You are NOT good for tanks.
I had to explicitly tell a Kritz Medic to stop giving me crits and give it to the Pyro instead.
When he did we went from losing by half the tank's HP to winning with time to spare.
You WILL pop Giant Medics if you snipe them.
Actually, Heavies are one of the worst classes about popping Uber Medics.
Learn to hold your fire.
You are not needed to slow down enemies. You do not need the Heater's fire; just Spycheck.
Brass Beast all the way; DPS is your best option.
Secondary is largely unimportant. I run a shotgun (the Business for lulz) to clean up stragglers.
Use GRU to get around. The speed outclasses the Steel.
Do note however that the GRU hits for less than 147 on crit and can't one shot Uber Medics.
This has actually been relevant in a game I played, but admittedly it should have been a non issue. Our Sniper was just bad.
Spirit is NOT good for tanks. It's plain not good. Don't use it. Heavy should never be punching the tank anyway.
KGB is okay if you can sneak a hit, but the best style of play makes it moot, so GRU it is.
Extremely viable exception: If you're buying loads of Resistances because you need to anyway, you can not only use the KGB safer but will gain more crits that way since you're not buying Crit canteens as often.
Buy Crits unless you're forced to play defensively, in which case you should buy Ubers.

General upgrade path is above; either focus on canteens or Resistances.
Offensive upgrades are generally a waste of money; Firing Speed in particularly is laughably bad.
Lastly, you're going to want Knockback Rage for boss enemies in the Mecha Engine stages. Save up for it.
If you're NOT playing Mecha Engine, Knockbage Rage is a huge waste of money.
There is nothing Knockback Rage can do that can't be done better by relying on your team mates to do their job so you can better spend that money on doing yours.

Always have the right canteen on you. You do NOT need Ammo Refill. Just pull out your GRU and run to the ammo.

Solid teams don't get into situations worth using the Pomson in, so the Rescue Ranger is better.
In fact, the Rescue Ranger makes tasks like bringing buildings along with the team while following tanks or regrouping extremely easy compared to usual.
It can even make dealing with Sentry Busters not only easier, but reduce the recovery time to increase damage output as well.
Pootispenser at first corner of main path on the side that the bots will turn past.
For teleporters, you want to put the entrance inside of a corner RIGHT NEAR an exit of the spawn, so it can be used as quickly as possible.
Some maps seems to favour which side you spawn at; for example, Bigrock players almost alway come out of the right doorway.
Sometimes the team will get trapped in the spawn after being overwhelmed and will need to teleport behind the enemy to deal with the problem.
If an allied Sniper does this they can usually clear the doorway easiest of anyone.
This pretty much requires the teleporter to be right by the spawn exit, and the teleporter exit should be such that it is safe to use regardless of whether the bots are behind the front line.
However, the exit should also be such that it requires minimal walking to return to the front line in the event that it is still being held (as it should be).
To give an example, again, this is probably the corner between the fence and the big rock the map is likely named after.
That rock in particular is a good place to put the dispenser if you can get it on top of the rock as it will be out of reach for the bots (mostly) and can fuel the entire team simultaneously.
In Mannworks, if the bots are going right, you can put the dispenser behind the wooden planks and let your team mates fire through them using the game's weird clipping physics.
For some waves, however, it's better to build around the corner's bend or more in the path to create a wall out of the players.
For Mannslaughter in particular, it can be most effective to build on top of the battlements for the first wave.
Wrangler all the way.
Wrangle down the main path aiming at the bot spawn, but if there's Giant Scouts, use the sentry as a wall at a chokepoint.
The more narrow the path, the better the sentry placement for blocking Giant Scouts.
Can be effective against other Giants, too, including the bosses.
If there are no Giant Scouts or other needs to use the sentry as a literal wall, it may be better to place it near an ammo crate in an elevated area toward the back (in other words, near where your Sniper might be).
The sentry shouldn't be too close to the dispenser anyway so that nothing bad happens to your team if you have trouble vacating from a Sentry Buster.
In addition keeping the sentry out of the bots' range while still being able to shoot back using the Wrangler is just an obvious strategy.
Wrangler cooldown can be canceled by replacing the building since it rebuilds instantly.
Wrangle everything, only stop when a Sentry Buster comes.
Wrangler shield and Building Health upgrades allow Engineer to Wrangle and only stop to heal the building in between enemies, so the Wrangling never stops.
Wrangle when Uber Medics are on the field (you should be Wrangling anyway) - do NOT go popping them.
Hold your fire if there's nothing to shoot but Uber Medics or their heal targets.
Jag is made moot and should probably be replaced with Southern or Default.

General upgrade path is Disp Range, Building Health, 2 way (Medics need this, perhaps others), Metal Capacity (allows instant rebuild of Sentry and Dispenser without stopping for metal), Melee Speed

You should be using Building Upgrade canteens.
There are times when Crit canteens and Uber canteens are viable but if your team plays well you should just invest in the safety net of Upgrade canteens.
In fact, it's probably faster, especially because of all the things that can go wrong when building normally, to run all the way back and buy an Upgrade canteen before returning to building where you intended in the event you don't already have a canteen for it.

Medic is my personal favourite class to play as in MvM, but its largest drawback is also the most painful of any there is.
Namely, a Medic can't do well unless they have a good ally to provide ubers to.
Medic can also be expensive, requiring lots of money for its key upgrades like Sniper (though not as many).
Medic also needs to be able to afford more canteens earlier on.
Thus, Medic works best if a Scout or other good money collecting option is part of the team.
The Kritzkrieg seems to be the optimal Medi Gun.
It facilitates Demomen between rounds, the damage is usually preferable (especially since Uber is useless against tanks) and the Medic can still share Uber with their heal target if necessary.
The boost, if measured in terms of the cost of respective canteens, is much more efficient than that of the Medi Gun.
However, sometimes the Kritzkrieg needs twice as many Uber canteens because another is necessary to safely return to the heal target after using one to Ubersaw for uber building (easily the only effective way to play Medic).
This is not often enough to warrant the Medi Gun being better, it seems.
The Vacc by comparison is just not powerful enough, since the regular Medi Gun provides full invincibility instead of partial while still being charged in time for critical situations just as reliably.
Medics can Ubersaw enemy Snipers (especially once they upgrade Movement Speed), Sentry Busters (an early combination of Melee Speed and Movement Speed combined with smart anticipation of its movement can allow for easy instant 4 hit ubers) and various other safe targets (melee Scouts, sometimes even the crit kind).
The Ubersaw can be used to build uber off of Bonk Scouts while they are invincible but can't counter attack, turning their advantage against them.
Gauntlet Heavies are particularly good for building free Ubers off of, but they may get in a hit now and then.
It helps to run in, cut, then run back to check your health, then stop to recover your health to full before continuing to avoid unnecessary endangerment.
When cutting, jump backward rather than simply moving backward when you do the "run" part of your hit and run.
This causes you to be knocked back further if you do get hit, pushing you further out of their range and keeping you safer.
Indicentally this same effect is good for knocking back Giants - if you get them into the air, a second or third knockback effect from an ally will be dramatically improved due to the bot no longer having friction by which to resist the effect.
Medic may benefit from bailing on an Ubersaw chain and returning to the heal target with less than full uber, then finishing building it by healing to avoid being in a compromising situation when their Uber canteen wears off.
Once a Medic has canteen sharing, they can share Uber with a heal target just before going to build uber with the Ubersaw to seamlessly transition from keeping their target invincible to ubering them with no worries of losing their ally in the mean time.
If necessary, they can then share another Uber canteen to keep the target alive while providing Kritz.
In the case of a Medi Gun, the Medic can pop as soon as they're full to survive the trip back to the heal target, often before the canteen even wears off.
The Quick pretty much has no place in MvM.
When sharing Crits to help with tanks after Ubersaw targets are gone, remember you can join in with your own Crits until it's time to share again.

General upgrade path is Ubersaw Speed, Ubercharge Duration, Canteen Sharing, then Movement Speed.

Uber canteens only, except to share Crits for tanks when there are no Ubersaw targets remaining.

First, let me say the most important tip for playing Sniper well that I take for granted after having played it so long: enable automatic rezoom in your client's advanced options.
The incredibly powerful Reload Speed upgrades for Sniper won't take full effect unless you have this option enabled.
It's important if there's a (good) Medic on your team to Slow Jarate Sentry Busters so that the Medic can Ubersaw them.
Sniper should use Jarate and Bushwacka. Crit things you've Jarated and crit tanks if near an active Buff Banner (unless your primary is stronger).
Mark things that get past you, mark things you need slowed down to headshot more easily, mark Giant Scouts to slow them.
Most importantly, you need to be firing a constant stream of headshots to fully use this class to its greatest potential.
Survey the map for the tightest cluster of robots, find the ones moving the slowest and shoot them first.
This is usually the bomb carrier because of their naturally reduced movement speed, and this also has the benefit of preventing the bomb carrier from leveling up too much, which is devastating without an Uber Medic killing class actively assassinating them (classes good for killing Uber Medics are also good for removing bomb level ups).
If no bots are moving slow enough, consider using Jarate with Slow to MAKE them slow enough.
Don't go burning your Jarate if you know Giant Scouts are coming, but you can usually do enough damage to a Giant to make using Jarate on the small bots before them worth it.
It's especially effective to Jarate a group of Pyro bots, because you might extinguish an ally just as they leave the range of their fire in the process.
Don't forget that you can headshot a Sentry Buster if you need to cause an explosion in a group of faster moving bots - the Buster may already have slow applied to it anyway if you have a friendly Scout on your team throwing Milk everywhere.

General upgrade path is Explosive, Slow, Reload Speed (most important upgrade, but at least one Explosive is needed for Sniper to truly shine), and Damage. Slow can be delayed in favour of Reload Speed.
Sniper can have trouble being autonomous due to usually not being near a dispenser. A Sniper using the Heatmaker will definitely want some Ammo Capacity upgrades (1 or 2 should be fine). Also consider grabbing at least one HP On Kill upgrade - even just one can dramatically recover HP when you kill an entire mob with a single Explosive Headshot.

Crit tanks and Uber when under pressure at times when you're most needed.

Now then, the Bargain and Heatmaker debate.
I've cleared every mission as Sniper using the Bargain.
I've recently switched back to trying the Heatmaker, which I recalled not liking in the past.
It certainly is a powerhouse in the right situation if Focus is built, but it burns ammo and thus requires a dispenser or ammo crate camping spot.
In addition, the Heatmaker is worse for Giant Medics and for safely dealing with Giants that would be better handled by peeking around corners with fully charged shots.
In the event there's nothing to build Focus off of, which is all too common, the Bargain wins then as well.
Small bots have low enough HP that spamming Explosive Headshots with the Heatmaker using nearby Giants is not much more effective than doing so with the Bargain.
Where the Heatmaker excels is trashing a group of Giants that are huddled together by causing nasty explosions at a rapid rate that have higher DPS than the headshots themselves.
However, after blasting one Giant that was right next to another, the remaining Giant didn't appear to be much worse off than it would normally be after being near a Bargain's blast.
To make matters worse, there are many times Giants are huddled as such with no small bots around to build focus off of, leaving the Heatmaker useless by comparison.
It seems that charged shots causing more damage also causes more hits from an explosion, which seems otherwise random.
Explosions seem to do 135 damage on average (apparently it's 150 before randomization and fall off are applied).
Sometimes the explosions go off twice and hit for 135 2 (sometimes more, perhaps?) times, which as was noted appears to be more likely when, if not entirely dependent upon, hitting with a higher damage shot, as in the case of a charged shot.
The odds of the damage hitting hard enough to kill nearby Medics rather than simply popping them also seems to be related to charging.
In either case, applying Jarate first will slow the targets down and mark them for team mates, as well as cause the explosions to mini-crit, guaranteeing instant kills of nearby Uber Medics and giving a solid 35% damage boost to the explosions.
In this case the Heatmaker has another advantage because it capitalizes on Explosive Headshots better, but again, this is painfully situational.
Ultimately, the Heatmaker is too unreliable to take the place of the Bargain as Mundy's best friend. You're welcome to try and disprove this, but I'll probably stick with my Giant Medic killing jezail.

Here's some numbers, to be updated as more tests are done.

Versus a Giant Demoknight with 30% move speed (default is 40%) and 90000 HP.
Map was mvm_coaltown_event; we fought it until it capped. Human error and differences in skill have altered the scores.
As there were no other bots, Focus was not relevant. Head count started at 0. Both rifles were fully charged for the first shot (which they would probably normally be anyway).

23185 Bargain
11622 Heatmaker

Despite all of the errors introduced, it should be pretty clear that the Bargain wins when there's only one Giant to fire at. Multiple natural runs of Wave 666 have already demonstrated that rifles that aren't the Heatmaker or Bargain just don't hold a candle to them and that the Huntsman is particularly bad, I don't have a log of the numbers, but I do have a log of the chat in which I spewed many profanities while giggling over how poor the other Sniper primaries did versus the Bargain. It's probably not appropriate to share

Here's some more testing done using 3 uber canteens for timing and a wall of cow cut-outs to corral the giants. Unlike with the tank section, timing ended when the third uber canteen was started, not when it was ended, since it's easier to listen for the canteen to start than look for its effects wearing off when sniping.

Note that no shots were missed during these tests. In the rare case I missed a headshot, I restarted the entire test. Human error has been almost completely removed.

1 Giant

Max Bargain (except Charge Rate), full charge 0 heads start
4662

Max Heatmaker (except Charge Rate), full charge start, no focus
3600

Max Heatmaker (except Charge Rate), full charge start, permanent focus
6000

2 Giants, same stats as above

Bargain
8411

7 head start made no difference using rapid quick scopes

Full charging for each shot with full charge start and 7 heads yielded
7805

Surprisingly not much different.

Heatmaker, focus
11250

Unsurprisingly not much better.

The Focus gives a rather poor boost considering how situational it is. The Heatmaker is still not going to help with Giant Medics and more importantly, for the same reason the Bargain is better for Giant Medics, the Bargain is also better for long streaks of giants that have no small bots to build Focus off of nearby and those inevitable situations when you just can't have Focus when you want it.

Nevermind that the Heatmaker is an ammo sink and pretty much requires you to be an ammo hog

Because of tanks I've been using the Revolver for its random crits and unhindered rate of fire.
However, Spy has no business dealing with tanks, so I personally may switch back to the Ambassador for 2 shotting enemy Snipers.
It has not been indicated that the Red saps better than the default.
As for knives, I was severely unimpressed with the Spycicle, which all too often would be useless after blocking fire which could have been stopped more efficiently with the Dead Ringer.
I've seen good things come from Spies who use the Big Earner but prefer the Kunai myself.
Originally I just used the default, and while it was nice actually having HP without having to backstab anything, building skill as Spy has allowed me to cope with the low HP start of the Kunai.
After getting the initial kill, it is easier to stab groups of sapped enemies while the bots that inevitably will avoid being sapped shoot at you, because the health stealing of the Kunai cancels the damage they deal at a rate almost comparable to using an Uber canteen.
The YER has obvious limitations due to preventing an initial disguise, which unlike the Kunai's initial low health is not an issue so easily resolved with skill.
For one thing, bots will know where you are and attack you pretty much at all times, especially if you are undisguised.
Approaching an enemy from behind won't let you "sneak up on them" without a disguise, making the YER difficult to use.
The Dead Ringer is much more appropriate for MvM, although in theory getting that first kill with the YER, pretty much only made possible by using the CnD, can lead to incredible killing sprees.
Unfortunately, these sprees would be useless against Giants, leaving the Spy unable to be useful against the greatest threat.
In addition to not being able to damage Giants using the YER and CnD build, the Spy would specifically not be able to attack Giant Uber Medics unless they bought Penetration, which would in general be a waste of money using the YER/CnD build.
Because Giant Medics don't face Spies, a Spy can easily 6 hit kill a Giant Medic, making them the absolute most effective class for taking them out.
When dealing with Engineer bots' nests, I personally find that stabbing and then Sapping is more effective, then shooting the teleporter.
Sapping first can cause the sapper to glitch and sap the Engineer but not the sentry.
Sometimes the sapper placed on the sentry will sap both, but it's not necessary or reliable.
Enemies like to come out of the teleporter while it's being shot, so sometimes I stab, Dead Ringer the sentry and then sap the teleporter, then leave allies to handle the sentry.
Mostly, though, the important thing is to hit the teleporter and the Engineer, and saving the sapper for the sentry by removing the Engineer first makes that easier.
When stabbing Giants, you may want to consider holding off your assault until the Giant has taken a few more steps if they are nearing a corner.
If you stab a Giant near enough to the corner, you may be able to stab them many times and retreat behind cover instantly, preserving your precious Dead Ringer usage.

General upgrade path is Sapper Radius, Pen, Knife Speed, then Movement Speed.

Ammo Refill if lots of things need to be sapped, Uber if lots of things need to be stabbed.
Crit tanks if that's all that's left to do.

Really the only person I can truly credit as of yet is myself. This sloppy, quickly thrown together guide (I'm told I type really fast if you're wondering how this extremely long guide is "sloppy and quick") was written by myself alone in a couple of hours and contains information that exclusively came from my own brain and the experiences that were stored in it.

However, there is a possibly growing list of exceptions that will be detailed below.

I've started peeking at other guides to see if I can get any ideas for things I've missed, either by reading things that are utterly wrong and should be called out for being so, or good tips that I use myself but either forgot to mention or just plain didn't know about (rare as that is).

So here's the (unfortunately short) list so far:

• Error, with his guide to being a useful MvM Sniper.
  
  http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=124646903
  
  The tidbits about targeting the bomb carrier, headshotting Sentry Busters and buying Ammo Capacity for the Heatmaker were obvious ideas I just plain forgot to mention despite that I would involuntarily follow such advice in an actual game.
  
  
• KrewL RaiN's Spy guide.
  
  http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=139615897
  
  It looks like a solid guide overall (I think I read a good half of it before my fear of forgetting to write things down settled in and I came rushing back to my own guide). While reading it I recalled that I forgot to mention Spy's effectiveness at backstabbing near corners in the Class Tips section - I had already done so in the Class Merits section, but that section isn't really where the reader is expected to find such information.

Help me out with this guide then. I need pictures. The guide looks boring without any.

You know what else I need? Proofreading. Not just for fixing mistakes or calling me out on obvious fluff, but for identifying and suggesting improvements for the many, MANY formatting nightmares. None of the incredible information in this guide will be of any use if you can't read it, and you probably can't because the formatting is HORRIBLE.

In fact, I know pretty much nothing about formatting these guides or how to view example source of existing guides to learn (which is how I would learn best; please don't spam me with links to guides about writing guides - that's too meta, even for me). If you want to make formatting improvement suggestions along with simple directions on how to implement them that'd be appreciated.