[Explained] The actual best Hive Scum build (H40)
 Hive Scum Build - Warhammer 40k: Darktide 

There are a ton of very powerful variants of this build, but this is the most optimized one. It's the single best melee DPS build in the game and it's flat out overpowered. I'll explain some of the less obvious choices and share some suggestions for customization. 

Hyper Critical + Hyper Violence

There's still a ton of confusion around these talents and how they interact. The bottom line is that the two nodes are NOT actually as anti-synergistic as many people argue. Hyper Critical doesn't reset Hyper Violence stacks. Into density, you won't actually have any problems keeping high damage stacks of Hyper Violence--especially with the Shivs. Additionally, Hyper Violence boosts Hyper Critical's output tremendously. It compounds with the insane modifiers from Rampage, Stimm Lab, Precognition, Float Like a Butterfly, etc., and allows Hyper Critical to cause very consistent one shots against annoying targets like Maulers, Mutants, Ragers, and Vanguards. Testing shows that combining both improves TTK against armor packs and mixed hordes without affecting TTK against ordinary hordes. It's also much more practical in a real game environment where you may not have the opportunity to set up optimal Hyper Violence stacks and just need to remove key targets quickly. 

If you really don't want to take Hyper Critical, a couple decent options include Channeled Aggression, Virulent Strain, and maybe Channeled Devastation, but truth be told, none of the available choices are nearly as impactful as Hyper Critical. 

Boom Bringer vs Chem Grenade

Boom Bringer is preferred because it's the more consistent problem solver. Chem Grenade has the potential to do more overall damage, but it's slower and less consistent in open areas--enemies will pathfind around it if they can. Still, Chem Grenade is a totally viable choice.

Rampage + Stimm Lab + Chemical Dependency Synergy

Rampage is the most broken ability of all time and this build both maximizes its value and gives it near permanent uptime. Boiled Blood and Pulverizing Strikes are mandatory for damage and uptime--you can swap out Forge's Bellow if you want to, but I'd strongly, strongly recommend keeping it. The stagger is incredibly useful for revives and it's very effective against both bosses and armor packs. It's almost as good as a Vet shout.

The Stimm + Chemical Dependency setup minimizes Rampage's Downtime by keeping the Stimm cooldown under 60 seconds in order to get the +40% Cooldown Regeneration from 4 stacks of Chemical Dependency with Maxed Out Chems. In other words, spam your Stimm the moment the mission starts and always maintain 3-4 stacks. 

For the Stimm itself, the Wildfire line is pound for pound the most valuable. Strength is the most valuable damage multiplier and finesse is almost as good as a base damage increase on this build in particular. This is because most of this build's damage comes from crits/weakspots.

The cooldown is only at 45 seconds. This makes it easy to keep 4 stacks of Chemical Dependency for an entire map, but if you'd like, you can put a couple points into the other lines. Keep in mind that this will require you to micro your Chemical Dependency stacks a bit more diligently, and you'll have less freedom to choose when you want to use it. Spur is the best line if you want to spend more points.

Also, Coated Weaponry + Splash Damage contribute to Sample Collector alongside your Needle Pistol shots, reducing the cooldown of your Stimm and letting you keep more uptime on its damage bonuses and your Chemical Dependency stacks.

Needle Pistol Perks

This is the standard Needle Pistol build. Reload Speed is chosen because none of the damage boosting perks change the Needle Pistol's breakpoints on Havoc 40. They do increase the TTK of the toxin, but the damage perks are only active when you're actually holding the Needle Pistol itself. You'll often take a few shots and then switch back to the Shivs, in which case the damage bonuses become irrelevant. The most common exceptions to this are when you're using the higher movement speed to kite a boss and when you're using Stripped Down against a gunner pack. Unyielding is useful in both cases for monstrosities and Reapers respectively. 

Curios

Gunner Damage Resistance isn't necessary because of Tis But a Scratch, Stripped Down, Burst of Energy, and Hive Scum's innate dodge angle buff. Bomber Damage Resistance is taken because it's the one type of damage that Hive Scum can't avoid. With a +3 Stamina Curio, the Hive Scum will never run out of stamina and gains an extra +6% attack speed from Swift Endurance. 

Common Picks You See on "Meta" Builds and Why You Probably Don't Want Them

-A Tertium Welcome/Punching Above One's Weight/Channeled Devastation/Toxic Mania. A 15% damage boost is quite small in the context of this build, which is getting 95% strength, 80% finesse, and has truly ungodly crit multipliers. None of them individually change breakpoints and taking more than one of them comes at the opportunity cost of better talents.

-Jittery isn't necessary with Hive Scum's insane stamina recovery speed and a +3 Stamina Curio. 

-Ruffian provides a significantly lower DPS increase than Anarchist for this specific crit build.

-Battering Momentum isn't necessary with the strength and cleave added from Rampage and Stimm Supply, as strength already increases cleave.

Final Notes

The combination of Street Tough, Burst of Energy, Tis But a Scratch, and Nimble make you extremely survivable. It's possible to swap Street Tough out for something else, but it does wonders for your consistency--it's a get out of jail free card every 8 seconds. Nimble is preferred over Alley Rat for melee builds because Alley Rat can make your spacing a bit awkward.