Vet stealth/mastermind build (read desc on why I think one of da best)
 Veteran Build - Warhammer 40k: Darktide 

I'll go over why I think Stealth is OP.
Chapter 1 - Why the build is good
Chapter 1.5 - Additional info
Chapter 2 - Talent/Nodes + Weapon explanation 

Chapter 1. Yes, VOC (Voice of Command) is busted, Golden toughness for all allies and stagger for all enemies in a 9 meter radius is insane, and you don't take any point tax grabbing demolition stockpile, but I still think this build is just as good. If not even better in certain situations.
Why? Simply because YOU are still in that situation, you're still swinging your powersword around trying to get a horde off of you. When your useful, much bigger and stronger teammates are right there, why should you be in that fight?
 
Keyword, useful, your teammates are pawns, tools, and baits to you, do NOT trust your teammates, I don't care how good they are, rely on yourself and plan for the worst, let me explain.
Infiltrate gives 8 seconds of complete invisibility, nearly nothing can harm you in this state, except for environmental hazards and Daemonhosts you somehow triggered, taking 'Low Profile' gives another 10 seconds of near-invisibility, giving a total of 18 seconds, I have seen hundreds of enemies just walk past me when I have this talent. This lets you do the RRK, Reposition (Yourself), Revive (Teammates), and Kill (Specialists). The eight second stage does end upon a significant action, eg, hitting an enemy (Still use the heavy attack tech for the knife that makes you faster if you think you're clear though!), but most importantly, reviving a teammate.  Reviving a teammate is not considered a 'Full action' by the game til they're fully revived, which means you have 10 seconds of invisibility again because you just revived a teammate, and have -90% less threat rating than them, letting you go revive another Teammate or take out Specialists.

Essentially, you can clutch any situation by just pressing F, since Stealth lets you be so versatile compared to VOC or Executioners Stance.
Also, you can use Inflitrate as a psuedo VOC since it replenishes all toughness and supresses all nearby enemies upon leaving stealth, so 

DO NOT DO NOT DO NOT BE AFRAID TO USE IT IF YOU'RE IN A BAD SITUATION, PLEASE!
Additional info: This build is not Plasma-gun specific, but it's recommended since Plasma gun is the best on Havoc 20-40 due to the cleave, but do not be afraid to use whatever you want to (Except for lasguns since we dont have shocktrooper or survivalist) 
Krak grenades are the most useful here since you can just drop 3 into any bosses weakpoint and deal massive damage to the boss, or take out Crushers, Bulwarks, or Maulers, other tanky enemies your team can't afford time for. 
If you're gonna swap off the knife for something else like a Powersword, Shovel, Combat. Axe, or Dueling Sword, take off 'Serrated Blade' since I added it to synergize with 'Flesh Tearer's blessing.
Curios can be whatever you want, but I recommend 2 Toughness + 1 Stamina curios, since I've had the best results with that myself, but I've had great results with +3 Toughness curious as well. Or health.. I guess.
The 5% Health and toughness refinements on the Curios are necessary, but the third refinement can be whatever you like.
I recommend 1 Gunner Resistance, and 2 Stamina Regens, why not 2 Gunners you ask? Because you are the only class in the game with a truly immediate response to Gunners, Snipers, etc. And you need the stamina regen to get OUT of situations. 

Chapter 2. Now to explain the nodes, I'll be going over the less-obvious ones exclusively, from top to bottom.
a.  Grenade tinkerer - This is one of the flex points of the build, so you can tick it off and delegate it to whatever you like, eg. 'Bring it down!', 'Rending Strikes', or 'Field Improvisation' if you're in need of extra support for your team.
b. Serrated Blade - Synergizes very well with 'Flesh Tearer', because Bleed Stacks are NOT a simple additive value. 2 Extra stacks of bleed can make big differences, guaranteeing kills on tankier enemies with only 1-2 stabs. But is a flex point, you can trade both Grenade Tinkerer and Serrated Blade for 'Always Prepared' and 'On your Toes' to buff your keystone, or like addendum a. Trade it for 'Bring it down!' or 'Rending strikes' 
c. 'Low profile' - Criminally underrated and I've explained why.
d (multiple) - Close Quarters Killzone, Surprise Attack, For The Emperor! Agile Engagement, Low Profile, and Weapons Specialist.
They all essentially stack at the same time upon using Infiltrate, You have crazy amounts of extra damage, are nigh invisible, high crit chance, full toughness, and your ability is already halfway recharged if you used all 8 seconds of true stealth, and then killing any specialists reduces that, technically giving it a shorter cooldown than VOC or Executioners Stance.
1. Knife, it's just good, one of the best survivability's in the game due to it's extreme movement and high backstab damage on anything, it's also really good for horde clear since with critical hits you apply 10 stacks of bleed to everything you hit (Essentially, you have 3 blessings on your knife due to serrated blade being the second level of 'Lacerate' essentially.), and you get tons of critical hits with the Knife. I reccomend the MK3 but you can use the MK6 if you like it.
2. Plasma Gun, the best Veteran weapon without any debate, near-infinite cleave lets you do your job the absolute best on Havoc 20-40, since you can hit Gunners, Shotgunners, Snipers, Trappers, etc through hordes to end the threat instantly, the other contender would be the boltgun or boltpistol but its up to you.

Conclusion: haha very good build lah, arguably the highest survivability in the game since you don't have to worry about surviving damage if you never take it in the first place, it lets you RRK, clutching games, nearly solo bosses or do objectives/puzzle games while invisible, and Infliltrate serves as a decent support tool by itself by buffing teammates in Coherency and Suppressing enemies (Stagger and suppression are a little different but idrk which one is better lmaooo)It is my absolute favorite build and I have saved dozens of runs with it, whether it was taking out specialists before they posed a threat, reviving the entire team, or making the impossible, possible (Heh, Dai-Gurren way..) by doing objectives or puzzles that couldn't be done to either of the two probelms above.
I don't know how to end tihs uhh jsafjnasfasfnsafjkasnfjk BYE!!