This guide is an extension of my guide to difficulty and classes:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3204106078

I will use the same approach in this guide to look at every perk from the three different player style perspectives.
This is important because not every perk fits every playstyle.

A marine earns a perk every time it reaches the next experience level.
There will be three random perks to choose from.
Sometimes a perk you want is not among them. Thats just how it is.
The choices are set in stone the moment you leave the mission and head back to the Otago.

(credit to steam user: GanjaCommando for finding a way to reload the options https://gtm.steamproxy.vip/app/1150440/discussions/0/6407003396122915023/ )

Avenger
Level 1
The squad earns 5% more Retaliation points.

Level 2
10%

Level 3
15%

My opinion
Does it even work? I recently tried out a Retaliation build with 5 marines having the trauma Pyrophobia level 3. Using flamethrower 5 times gives 225 stress and 10 stress levels total, and Retaliation is at max. 4 marines had Avenger level 3 and the last had Avenger level 2. That should have been 70% increase in Retaliation points but still takes 10 stress levels to max Retaliation. My conclusion is that it does not work, or if it works, it is tied to recieving damage to your marines and not from gaining stress levels.

Casual
What is Retaliation?

Experienced
Could be useful if it works.

Expert
Not working on stress level gains makes this useless.

Bold
Level 1
Increases bravery by 5 points

Level 2
10 points

Level 3
15 points

My opinion
This perk enhances the bravery stat of the marine wich in turn reduces incoming stress by a percentage. Depending on your playstyle this can be either good or bad. I recently played through the game with two gunners level 10+ and they get double dps when reaching stress level 3(Unleash Hell). This perk would make that happen slower. And there exists so many ways to control stress already. Only a playstyle that ignores the stress mechanic will benefit from this passive perk.

Casual
Useful. One less problem to worry about.

Experienced
Useless

Expert
Useless

Dead Eye
Level 1
Critical and dismemberment chances are increased by 13% for the marine

Level 2
25%

Level 3
38%

My opinion
This perk increases the overall dps of the marine. Most impactful on a gunner past level 6, since it gets the base increase of 1 point from the class atttribute Bone Breaker. Less impact on the rest of the squad, but I always try to fit it into the build regardless.

Casual
Yes

Experienced
Yes

Expert
Yes

Enduring
The Wound effect is applied at 1 hitpoints instead of 2.


My opinion
If you have marines taking lots of damage, this will have your marines fighting at full strength for one hit more. But the perk Tough does the same by giving the marine an extra hitpoint.

Casual
Useful

Experienced
Useless

Expert
Useless

Fast
The whole squad moves faster.
Tested on Dead Hills. Moved same distance over and over with different setup.
(Test data: 0xFast = 75 seconds, 1x Fast = 72 seconds, 2x Fast = 70 seconds, 3x Fast = 68 seconds, 4x Fast = 66 seconds, 5x Fast = 64 seconds, 5x Fast + Recon = 57 seconds)
Testing shows it is about 3.5% speed increase for each Fast perk.
Stacks with other marines having Fast.
Stacks with Recon class attribute Fast Deployment (about 15.5% speed increase)
Full squad of four marines with a level 6 Recon will give about 30% speed increase.

My opinion
If you like to play it nice and slow, then avoid this perk. But if every second counts then this perk is very useful. It really show its benefit right after a rest or when exiting an elevator and the aliens have yet to spawn. You can manage to storm a mission objective without seeing any alien. Also after a hunt has just begun, it makes you reach the shelter to weld and stop the hunt or the elevator for the same effect. This perk has the potential to drastically slow the aggressiveness counter.

Casual
The marines stop shooting when they run, you know.

Experienced
Already have a Recon, does not need more speed.

Expert
Yes

Hardened
Level 1
Increases the max armor of the marine with 1 point
The default is 2 points of armor, this will raise it to 3 points.

Level 2
2 points
The default is 2 points of armor, this will raise it to 4 points.

Level 3
3 points
The default is 2 points of armor, this will raise it to 5 points.

My opinion
Once lost, the armor does not come back for the rest of the deployment. So it is like temporary hitpoints in a way. Although the marine needs at least 1 point of armor for the acid resistance to work. Taking armor damage will also add to time in medbay. This perk is best for a playstyle that very often kill aliens at shotgun range or very close but not reaching the marines, as it would protect against the acid. If aliens are in melee all the time, then this perk is not so useful as the armor would be gone in no time.

Casual
Armor? I only see the red bar.

Experienced
Useful for those close encounters

Expert
Useless

Looter
Level 1
The squad earns 10% more material when a marine loots a material crate.

Level 2
20%

Level 3
30%

My opinion
I took this perk on my first playthrough, thinking it would be really good on my B-team to make a second deployment and grab all the material crates left behind by the A-team. But on my next playthrough I realized this perk was useless. In the start of the campaign, before the Death Clock starts, you can skip as many days you want. The only consequence is that the planet infestation increases to 5. But as I already adviced on difficulty settings, this should always be set at 5 from the start. And there exists a day event called Expedition, which gives a random reward(50 materials, 10 alien DNA, 1 engineer, 1 physician or a new level 1 marine.). Also every day each engineer gives you 5 materials. So by skipping lots of days you will be rich. I have tested skipping huge ammount of days(500+ days), and there does not seem to be any consequences. Unless you refuse to skip days, this perk is obsolete.

Casual
Useless

Experienced
Useless

Expert
Useless

Mind of Steel
Marine automatically heals 1 trauma point per campagn turn.

My opinion
Trauma points comes from having stress reach above 100% during a mission, or getting 3 points in a trauma from a day event. On my last playthrough I did a Retaliation build with Pyrophobia on every marine. Having this perk would have been a disaster. There is only one bad trauma, Obsessive, which increases reload times by 50%. The rest of the traumas just increase stress gain under certain conditions. Useful for the player styles that do not handle stress very vell, meaning having a close eye on what stress debuff is applied and removing the unwanted ones.

Casual
Useful

Experienced
Useful

Expert
Useless

Quick Hands
Reloading times for this marine is 50% faster (Applies to both primary and secondary weapons)

My opinion
Reloading times differ for each weapon. From 3 seconds on the Pulse Rifle to 5 seconds on the Smart Gun. Reducing this to 1.5 seconds and 2.5 seconds is really good when action is closing in. A marine busy reloading is not slowing aliens with the Suppressive Fire skill, the slow effect is disabled while reloading. This can be somewhat countered by having two marines using Suppressive fire on the same area, but double cost to command points. Every playstyle should try to fit this perk into their build.

Casual
Yes

Experienced
Yes

Expert
Yes

Redemption
Removes the marines negative trait.

My opinion
It hurts having to take this perk just to remove a negative trait. Feels wasted. But some negative traits are really bad. But not to be taken on marines that increase your stress if you are aiming for a Retaliation build. The main negative traits to remove are those that reduce dps. Some traits are just annoyances that you can live with, so consider carefully before taking this perk.

Casual
Only take it if marine has one of these negative traits:

• Bully
  
• Gloomy
  
• Jinx
  
• Poor Health
  
• Poor Sight
  
• Rebel
  
• Tense
  
• Rusty (If marine lacks the perk Unbreakable Will)
  
• Unfit

Experienced
Only take it if marine has one of these negative traits:

• Bully
  
• Clumsy
  
• Gloomy
  
• Heavy Heart
  
• Jinx
  
• Poor Health
  
• Poor Sight
  
• Rebel
  
• Rusty (If marine lacks the perk Unbreakable Will)
  
• Tense
  
• Undisciplined

Expert
Only take it if marine has one of these negative traits:

• Brawler
  
• Clumsy
  
• Heavy Heart
  
• Jinx
  
• Poor Health
  
• Poor Sight
  
• Rebel
  
• Rusty (If marine lacks the perk Unbreakable Will)
  
• Tense
  
• Undisciplined

Resourceful
Provides 1 ammo clip after each rest.

My opinion
This depends on both style of play and what build you are doing for the playthrough. If you are doing a stealthy approach, and therefore fighting much less, then this perk becomes rather useless. So the more you need to shoot the better this perk gets. Also this is the best perk for those tough missions where you have to deploy multiple times. Having it on every marine will solve your ammo problems, if you have pouches on every marine and bring 10 tools, you could rest 4 times and have 20 ammo clips. Very nice perk.

Casual
Yes

Experienced
Yes

Expert
Depends on the build

Sharp Shooter
Level 1
Increases the marine's max accuracy by 5 points
Default accuracy is 50%, this raises accuracy to 55%

Level 2
10 points
Default accuracy is 50%, this raises accuracy to 60%

Level 3
15 points
Default accuracy is 50%, this raises accuracy to 65%

My opinion
This is maybe the best perk in the game. Direct dps increase. Should be taken on every marine and every build. It stacks with the common upgrade Aiming Sight, for a max accuracy of 70%.

Casual
Yes

Experienced
Yes

Expert
Yes

Smart Ass
Grants access to the Hack interaction. The marine has the ability to unlock encrypted doors.
This is the same as the level 3 class attribute of the tecker.

My opinion
This perk makes the tecker less useful. There is not enough encrypted doors for the tool cost to be any problem. Having this perk on two marines makes it more flexible. A must take for those playing without a tecker in the squad.

Casual
Yes, have it on at least one marine.

Experienced
Yes, have it on at least one marine. Or one in addition to the tecker.

Expert
Yes, have it on at least one marine.

Team Spirit
Increases max command points by 1 for every two marines with this perk.

My opinion
If you like to use lots of skills then you need a large pool of command points. Having this on 4 marines will give you 2 command points extra. This stacks with the sergeants Radio Backpack upgrade. Very nice skill.

Casual
Skills?

Experienced
Yes, have it on 4 marines

Expert
Yes, have it on 4 marines

Tough
Level 1
Increases the marines maximum hitpoints by 1
Default is 6 hitpoints, this raises it to 7 hitpoints.

Level 2
2
Default is 6 hitpoints, this raises it to 8 hitpoints.

Level 3
3
Default is 6 hitpoints, this raises it to 9 hitpoints.

My opinion
On default alien damage settings, this perk is quite powerful. Also with higher hitpoints the danger of a marine reaching 2 hitpoints and getting a random wound takes longer. On higher difficulty it might be worth to take one level just so the marine is outside of instakill range.

Casual
Yes

Experienced
Better to invest in Hardened.

Expert
Maybe 1 level if playing on +5 alien damage, otherwise skip.

Unbreakable Will
The marine is no longer subject to tiredness effect.
This makes the marine immune to both tired and exausted statuses.
Day events making the marine exausted will have no effect.

My opinion
Nobody wants to deploy tired marines and have the 15 penalty to accuracy.
So the question becomes, do you have time to spend an extra day between deployments to rest?
The answer is that it depends on how many deployments you need per mission. And it depends on the Death Clock, is it disabled? I have played on nightmare difficulty and without this perk, and used an extra day between missions to rest, and ended up with 6 days left on the Death Clock. But that was without having the need to deploy multiple times. There exists one mission where you do not have time to rest, but it is rather easy.

Casual
Only if Death Clock is active.

Experienced
Yes

Expert
Useless

Having gone through every perk in the game, I will now make a ranked list for every playstyle and taking into accord the difficulty I adviced for each playstyle earlier.
When choosing perks, my advice is to start from the top of my list and select the highest perk.

Casual

1. Sharp Shooter
   
2. Redemption (See Redemption section of this guide)
   
3. Smart Ass (on at least one marine)
   
4. Tough
   
5. Quick Hands
   
6. Dead Eye
   
7. Resourceful
   
8. Enduring
   
9. Mind of Steel
   
10. Bold

Experienced

1. Unbreakable Will
   
2. Redemption (See Redemption section of this guide)
   
3. Sharp Shooter
   
4. Smart Ass (on at least one marine)
   
5. Quick Hands
   
6. Team Spirit (on 4 marines)
   
7. Dead Eye
   
8. Resourceful
   
9. Mind of Steel
   
10. Hardened

Expert

1. Sharp Shooter
   
2. Redemption (See Redemption section of this guide)
   
3. Team Spirit (on 4 marines)
   
4. Smart Ass (on at least one marine)
   
5. Quick Hands
   
6. Fast
   
7. Dead Eye
   
8. Resourceful
   
9. Tough

Happy gaming!