Stimm Share Scum
 Hive Scum Build - Warhammer 40k: Darktide 

This build is designed around a bug that currently exists with Stimm Supply: if you drop your stimm crate while the cartel stimm is on cooldown, you will still get the stimm buff if you leave and re-enter the ground effect. This allows you to use a 75 cooldown stimm and still have full Chemical Dependency uptime with Maxed Out Chems. Fast Acting Stimms is basically required for any Stimm Supply build, but Practiced Deployment and Sample Collector work together to significantly increase stimm uptime in this build. Booby Trap is decent, but I don't think it's worth the point in this build since we have longer CDs.

You want to start by manually using the cartel stimm on yourself, then drop Stimm Supply once your stimm buff wears off. Don't forget to dodge in and out of the crate radius before it expires. Try to place the crate ahead of where teammates will walk so they get the buff even if they don't know to leave and re-enter. Getting kills will reduce the stimm cooldown, which then resets your ability CD, and then you repeat the cycle. If your ability comes off CD before your stimm (which will usually happen unless you're spamming toxin), just drop the crate immediately so you can refresh your buff before the stimm CD ends. If you know you'll be facing corruption damage soon (e.g. daemonhost, hound wave, burster wave, pox gas) then save your ability for the appropriate time to give your team immunity.

It seems that you can gain double the bonuses of the cartel stimm by manually using it before activating the ability, but this is overkill in most cases and will make it harder to maintain Chem Dep stacks. It could be worth doing for a boss fight though.

Melee weapon is personal preference. I've been having success with the Mk VI knife because it has good one-shot breakpoints with Hyper-Critical and it has good cleave on heavy attacks without needing any cleave bonuses in the build. Shivs have a lot of the same advantages as the knife with a slightly different playstyle, and can use the same perks/blessings. Devil's Claw sword is a great option since is mulches hordes and is strong against bosses, while its weakness against armor is covered by the needle pistol. I recommend Catachan Mk VII "Devil's Claw" Sword with Shred and Rampage blessings, and damage vs flak + unyielding perks. With Devil's Claw, swap out The Sweet Spot for Battering Strikes.

I usually recommend Riposte on the knife, but in this build it would exceed 100% crit chance when all buffs are active. You won't actually have all buffs active most of the time, so Riposte is still worth considering for more consistent damage. I went with Flesh Tearer instead for some DoT DPS; the knife still reaches up to 90% crit chance with all buffs active, so you will be applying bleed on most hits. However, from my testing with scoreboard, bleeding usually accounts for a small percentage of your total damage/kills if you have a decent team. Bleed is strong for clutching and for soloing monstrosities/bosses though. Precognition is another good option for the second blessing, but it tends to have low uptime due to the 2 second duration.

Needle pistol is the best-in-slot ranged weapon for this build. Not only is it strong against all relevant enemy types, but the toxin kills also reduce your cooldowns through Sample Collector. Both versions of the pistol are good, but the Mk II gives you the option of spreading toxin for horde clear with the special action secondary mode.

Most blessings on the needle pistol are not practical. Run 'n' Gun is the most useful because it increases the gun's accuracy. Stripped Down gives protection from ranged attacks while sprinting/slide-spamming as long as you maintain your stamina. Damage perks do affect the toxin, but on most targets it will just make them die slightly faster and won't change how many shots you fire in a realistic combat, so I just take damage vs unyielding for reapers and bosses, along with reload speed for less downtime. There's probably a case for using a second damage perk instead of reload speed, but I haven't done enough testing to say for sure. Damage stat on the needle pistol does almost nothing, so it's the best dump stat.

In the stimm lab, I focus on strength + crit chance since they have high value across classes, weapons, and enemy types. I take 3 nodes in both the attack speed and toughness branches because these are universally helpful and relatively cost-effective. That leaves 3 points leftover that can only go to the cooldown branch, which still gives a decent 12.5% CDR for your team.

For the blitz, you can take either Chem Grenade or Boom Bringer. I like Chem Grenade for the synergy with Sample Collector, but Boom Bringer is stronger in specific situations (i.e. against crusher packs and monstrosities). Chem grenades are also easier to get value from if you need to blow your extras before taking a grenade pickup.

You can swap the +3 stamina curio for +17% toughness or +21% health if you prefer. I like having extra stamina on the knife for push attacks, not to mention the attack speed from Swift Endurance.