Orks are the main baddie in campaign, and often the op force in multiplayer. They are not as consistent as the other two fractions. They have above average firepower balanced by low accuracy, although on occasion they will hit, a lot, and it will be painful.

Infantry
Ork infantry are a perfect display of what I just said. They have powerful weapons, great numbers, and have worse accuracy than stormtroopers in the begining of Star Wars: A New Hope. Most are very cost efficient, but at the same time suffer heavy losses so that experience will not be gained. In campaign they make up at least 50% of the enemy forces, so killing them will be commonplace.

Walkers
Orks have a wide variety of walkers. Usually they're best vs infantry but they can hurt your tanks if you let them get into melee. They're not so useful in multiplayer due to their cost, but if you start with them may as well use them.

Vehicles
Ork vehicles range from jeeps with guns strapped on to medium armor. You can tell what they do usually from the weapon listed in their name.

Tanks
Ork tanks are interesting. The meduim armor is near useless because of lack of guns. Their heavy tanks range from useless expensive crap to useful expensive ow. Their "normal/standard" suuperheavy the Skullhamma is probably the best one in game due the combination of speed, firepower and versitility. The other superheavy type, the Battlewagon, is not as good or useful, but make decent support or brick units when you have them.

Artillery
Funnily enough, the Orks have only one officaly mobile arty piece, with the others being stationary target practice. Its missiles are flown by grots, with accuracy to be expected of such pilots, abysmal.

Titans
Ork titans are probably better than Steel Legion ones. 1v1 the Gargant beats a Reaver, but is worse vs infantry. It also still has problems vs shadowswords. Stompas range from terrifying to decidedly meh depending on type.

Warboss
-Spotting 4, Move 4
-very important unit lorewise and in gameplay
-One of few units with good vision makes it a must, also has range 3 weapons (another uncommon thing for infantry)
-Tougher than normal orks and each armed with Big Shootas, the ork equiv to the Heavy Bolter.

Ghazghull Thraka
-Spotting 4, Move 4
-combination of Warboss and Mega Armored Nobs
-Vision 4 and has double Big Shootas, as well as a decent melee attack
-Very cost inefficient

Mega Armored Nobz
-Spotting 3, Move 3
-Very durable inexpensive unit with a lot of anti-infantry firepower
-Only good vs medium+ armor in melee
-Shock Infantry

Nobz
-Spotting 2, Move 2
-Armed with the shoota, a bolter equivalent
-Can engage medium armor from cover, and can hurt infantry and light armor in melee

Stormboyz
-Spotting 2, Move 5
-Fairly dangerous to armor in melee
-Only a weak range 1 shooting ability, although can hurt infantry if not weakened first

Ard Boyz with Sluggas
-Spotting 2, Move 3
-Armored Infantry, 1.5x as tough as normal brethren
-Close range assault infantry
-Will engage Steel Legion infantry favorably, and if gets into melee will hurt normal space marines

Ard Boyz with Shootas
-Spotting 2, Move 3
-Armored medium range infantry, a threat to medium armor or weaker targets

Slugga Boyz
-Spotting 2, Move 3
-Cheap expendable assault infantry

Shoota Boyz
-Spotting 2, Move 3
-Cheap expendable screening infantry
-Can weaken almost any “regular” unit

Shoota Boyz with Big Shootas
-Spotting 2, Movement 3
-Most expensive infantry unit par Ghazghkull Thraka
-Only Support unit orks have
-Firepower is mediocre, but so many shots will hurt anything medium armor or weaker

Burna Boyz
-Spotting 2, Move 3
-Devastating short range unit
-The flamethrower “burna” is fairly damaging and should be avoided (or used) if possible
-Again though, medium armor or weaker are their targets

Kommandoz
-Spotting 2, Move 4
-One of the few true threats to armor amoung ork infantry
-Close range assault unit
-Does not perform better than Burnas when fighting infantry, should be preserved for rushes against tanks if you’re going to use them

TankBustas
-Spotting 2, Move 3
-Good anti-tank unit with a powerful range 3 missile and a devastating anti-tank melee unit
-Does respectable damage vs infantry at range, but there are better options
-One of the units to fear the most in campaign, and use most in multiplayer (provided it has not turned to superheavy spam)
-Can do heavy damage to superheavies, or even titans if they close to melee (esp. considering their cost). 3 can kill a superheavy, 4 can kill a titan

Lootas
-Spotting 2, Move 3
-Medium range, powerful shooting unit, to be avoided if in cover
-Best vs medium armor or lower, will damage superheavies if they close the range
-Next to toothless vs titans

Gretchin
-Spotting 1, Move 2
-Bullet Bait
-Can hurt infantry through sheer volume of shots
-Die to a breeze, more shots you fire at them the better. Shot strength will not matter

Killa Kans with Skorcha
-Spotting 2, Move 4
-Anti-infantry melee unit
-Crusha does decent vs armor
-Will be shot and killed from range often as only has range 0

Killa Kans with Big Shootas
-Spotting 2, Move 4
-Better melee weapon for ant-tank duties
-Big Shoota lets it engage infantry from range

Deff Dread with Big Shootas
-Spotting 2, Move 4
-More durable than Killa kans
-Anti-Infantry walker, 2 crushas have same stats as 1
-Less overall firepower than kans, but loses firepower slower

Deff Dread with Kill Saw
-Spotting 2, Move 4
-Powerful melee only Deff Dread, good vs all targets
-Will get shot in the face by lascannons

Mega Dread
-Spotting 2, Move 3
-Anti-tank walker
-Bit of ranged, bit of melee
-Low Unit Strength hurts firepower significantly, but is very durable

Mega Dread with Killkannons
-Spotting 2, Move 3
-Exchanged melee attack for 2x the ranged firepower
-Still won’t kill much, but will hurt things (it’ll do 1 or 2 damage, but rarely kill a midel w/o help)

Mega Dread with Rippa Klaws
-Spotting 2, Move 3
-Does more damage than the other 2 mega dreads, but melee only has its typical problems

Gargantuan Squiggoth with Killkannon
-Spotting 2, Move 3
-Decent firepower per model, but low unit strength overall makes it too expensive for too little firepower
-Toughest walker, makes a good road block

Gargantuan Squiggoth with Supa-Zzap Gun
-Spotting 2, Move 3
-Better “main gun” than other version, for the same cost
-Firepower more worth it

Squiggoth with Kannon
-Spotting 2, Move 3
-Fairly durable unit, but not the best way to get a lot of kannons
-Decent anti-vehicle support unit of no other option to pick

Squiggoth with Supa-loba
-Spotting 2, Move 3
-Decent artillery support walker

Squiggoth with Zzap Gun
-Spotting 2, Move 3
-Trades the kannon’s ap for more strength
-Better vs medium and light armor

Squiggoth with Flakka-Dakka Gun
-Spotting 2, Move 3
-Anemic firepower made useful only by how fragile fliers are
-Given how easy it is at the moment to hit fliers this unit is near worthless

Warbiker Mob
-Spotting 4, Move 7
-Fast scout unit
-Almost useless in a fire fight, only decent vs Infantry out of cover

Warbuggy with Rokkits
-Spotting 4, Move 7
-Scout unit with quite a bit of ant-tank firepower
-Still fragile, so expect losses when attacking with or against

Warbuggy with Big Shoota
-Spotting 4, Move 7
-Scout units with good anti-infantry firepower
-Since it should be fighting infantry, and hopefully not tanks it’ll last long as intended purpose, but will do little damage if not fighting infantry

Wartrakk with Big Shoota
-Spotting 1, Move 5
-Trades scouting ability of the warbuggy and a slight amount of firepower for slightly better staying power

Wartrakk with Skorcha
-Spotting 1, Move 5
-Melee version of previous
-2x shots, but harder to apply on desired target and won’t retaliate most of the time

Wartrakk with Rokkits
-Spotting 1, Move 5
-Significantly less firepower than buggy, with less than half the shots for a higher cost

Gun Trukk with Kannon
-Spotting 1, Move 5
-Good firepower anti-tank fire support unit

Gun Trukk with with Big Zzappa
-Spotting 1, Move 5
-Anti-tank unit that can easily hurt superheavies, if it ever hit…

Gun Trukk with Big Lobba
-Spotting 1, Move 5
-Light artillery support
-Better Artillery options exist

Big Trakk with Flakka-Dakka Gun
-Spotting 2, Move 4
-Medium Armor, inferior to Leman Russ firepower, but about as durable
-Good AA firepower, but vs ground targets it tends to be lacking
-Best used as anti-infantry most of the time
-Any tank can waltz up to and kill it

Big Trakk with Big Shootas
-Spotting 2, Move 4
-Slightly less shots but better strength anti-infantry
-Overall results are actually worse than Flakka-Dakka gun variant

Big Trakk with Supa-Kannon
-Spotting 2, Move 4
-Arty support, roughly equivalent to a Basilisk, if the Basilisk had trouble hitting a barn

Big Trakk with Zzap Gun
-Spotting 2, Move 4
-High strength anti-light/medium armor

Big Trakk with Killakannon
-Spotting 2, Move 4
-Same strength as Zzap variant, but with decent armor piercing

Grot Tank
-Spotting 2, Move 5
-Cheapish light tank
-Weapons are effective vs medium armor and infantry, but low accuracy means it’ll rarely hit

Gunwagon with Zzap Gun
-Spotting 2, Move 5
-Medium armor with 7 unit strength
-Only has a zzap gun, which is effective vs non-heavy armor, but low accuracy means hits are rare

Gunwagon with Kannon
-Spotting 2, Move 5
-Only has a kannon, decent anti-meduim armor firepower, but low accuracy

Grot Mega Tank
-Spotting 2, Move 5
-Fairly powerful medium armor
-Best at range 1 were it’ll fire with all weapons and has best accuracy
-Best killed from range, where it’ll barely hit and only fire a few weapons

Kill Krusha
-Spotting 2, Move 3
-Heavy armor effective vs all targets
-Low unit strength/shot count limits firepower, although has high accuracy

Kill Krusha (upgraded)
-Spotting 2, Move 4
-Slightly more anti-infantry firepower , defense and better initiative for a small cost increase

Kill Blasta
-Spotting 2, Move 4
-Powerful anti-infantry heavy tank
-Main gun is roughly equivalent to the mega bolter
-Mediocre vs medium armor and is killed fairly easily by heavy armor

Kill Blasta (upgraded)
-Spotting 2, Move 4
-Better anti-infantry, initiative and defense for a small cost increase

Kill Bursta
-Spotting 2, Move 4
-Sports a powerful Belly Gun which is excellent vs all targets
-Although killed fairly easily by superheavies and titans, it will hit back hard

Kill Bursta (upgraded)
-Spotting 2, Move 4
-Higher Initiative and better anti-infantry and small defense boost
-Helps more so with this unit, as it’ll match or best titan initiative (no veterancy), letting it do heavy damage if engaged from close range
-Best to kill with arty, or with upgraded superheavies with their higher initiative

Skullhamma Battlefortress
-Spotting 2, Move 5
-Fast powerful superheavy tank
-Outperforms the Baneblade outside of the demolisher cannon’s range
-For 10 more requisition outperforms the Kill Krusha (upgraded) in every way
-Overall an incredible unit/pita

Deathrolla Battlefortress
-Spotting 2, Move 3
-Superheavy medium armor killer with an assault attack
-Same cost as Skullhamma means you should never really use this

Battlefortress with Supa-Kannon
-Spotting 2, Move 3
-Trades some anti-medium armor ability for an artillery kannon
-Low unit strength means the arty gun underperforms

Flakk Battlefortress
-Spotting 2, Move 3
-Support super heavy with AA and arty attacks
-Cannot directly engage armor
-Generally lackluster for cost, better off with Battlefortress with Supa-Kanon or Skullhamma unless fliers change

Grot Bomb Launcher
-Spotting 1, Move 4
-Decent strength, with decent ap
-Good anti-tank artillery

All other arty
-Spotting 1, Move 0
-Shouldn’t really be used in multiplayer
-Exploitable range

Deffkopta
-Spotting 3, Move 6
-Fragile unit with a powerful anti-tank attack that also works well vs space marine infantry
-Best used as fire support, preferably against units that have limited range

Deffkkopta with Buzzsaw
-Spotting 3, Move 6
-Adds a powerful anti-tank melee attack
-Hard to use the melee attack due to losses when attacking from so close
-Will heavily damage your tanks in the missions it appears, best dealt with via infantry (even conscripts hurt them pretty badly)

Stompa
-Spotting 2, Move 3
-Weakest titan the Orks have
-Ranged fighting ability tied with Warhound Scout titan, mainly comes down to who attacks who
-Melee attack adds to anti-tank firepower and will hurt superheabies
-Not so good vs infantry

Stompa with Belly Gun
-Ranged support Stompa
-Devastating firepower vs armor, will typically win vs Warhound scout titans, and hurt Reavers badly but lose

Goff Klawstompa
-Worst Ork titan in my opinion
-Melee attack can be good vs superheavies, but it losses all anti-tank firepower
-Does ok vs infantry, but ok really isn’t enough for a titan considering its lack of anti-tank
-Does have slight HP and defense boost over normal stompas

Big Mek’s Stompa
-Fairly powerful anti-infantry firepower
-Can do decent vs armor due to the Lifta-Droppa’s 50% ap and the Gaze of Mork, not very reliable though
-Even tougher than the Goff Klawstompa

Gargant
-Biggest Baddest Ork Titan
-As durable as a Reaver titan, and typically bests the Reaver
-Only a shadowsword really poses a grave threat

Orks units can typically be seperated into either stellar or trash. This can probably be tiedto thier costs not being as well balanced as the campaign fractions. This section basically lists the units I liked best in multiplayer and why.

Mega Armored Nobz
-They're only 250 requistion, and can kill any infantry not terminators. You could actually exchange these 3-1 vs terminators with assault cannons or missles and come out on top economically. They also tend to soak a lot of punishment making them excelent bullet bait. They work excellently as a screen due to their good vision.

Tankbustas
-Usually when the enemy sees you titan units, or in anticipation of they bring shadowswords, other superheavy tanks and Destoyers or Vanquisher cannon units. Tankbustas hard counter them. If the tankbustas can reach melee they'll oftend do 6+ damage to a superheavy unit. Even at range they'll put a few points in. Not too shabby for harassing infantry at range 3 either. Overall very useful for only 180 requisition.

Kill Busta
-Preferably the upgraded variant, this heavy tank is a shadowsword for the Orks. The gun has less power, but more shots on a unit at full strength and the same 50% ap making this unit very handy, if a bit on the fragile side.

Kill Blasta
-I don't use this as often due to how quickly fight devolve into tank fights, but honourable mention for its great anti-infantry firepower

Skullhamma
-Probably the best superheavy for cost. Only other superheavy that it really has to watch out for are shadowswords, which your tankbustas should be dealing with. It can easily close and avoid the demolisher cannon of the Baneblade, or dance out of the range of normal tanks and shell from afar. It can bully less tanks and most infantry, as well as having very high accuracy for Orks. Costs only a little more than a Tank Busta/Blasta. Destroyers tend to be a pesky problem, and probably not engaged unless you have to.

Stompa with Belly Gun
-Just overall easier to use and do damage with than normal stompa. Its firepower is pretty scary.

Gargant
-Wins 1v1 with a Reaver most of the time. Although it lacks the ability to deal with infantry, it usually has nothing to fear. Only shadowswords can kill this thing "easily", anddestoryers will do it cost effectively.