Most Fun I've had with this game
 Hive Scum Build - Warhammer 40k: Darktide 

So this Build is my favourite one in the game, but what can it do and how does it play?

With this build you're incredibly mobile and able to react to any situation. You can play as a support role, or clutch and solo as you see fit.

How does it achieve this?

With the Branx MkVI Needle Pistol, Run 'n' Gun, Stripped Down and Slippery Customer you have Quartett of movement speed while being able to tag enemies in passing. This lets you disengage situations easily and Battery carry missions basically get solved on their own.

In more sticky moments you have your trusty 6 Blackout Grenades with Pocket Toxin, use this to get out of being surrounded, or to disrupt Crusher ushers and Bulwark ks. These are also perfect for saving teammates when they're swarmed or run headfirst into a group of Ragers.

For picking people up you have the great one to punch of 1. Blackout and 2. Booby Trap. This way you clear the area and your Stim explodes after 5 seconds, right when your rezzing and the enemies want to swarm again.

With Stimm Supply and all it's nodes active your Stim effect lasts for 20 seconds (5 seconds Area+15 after explosion) with a cooldown of 27 seconds that starts immediately when you drop the supply.
This means you can have your Stims almost always active while constantly supplying you with disrupting and poisonous bombs you can strategically place.

Now for the Stim itself: 8% Power, 8% Attack Speed, 50% Weapon Swap Speed and -15% Stamina Cost
This not only gives a near constant very noticable boost to your whole team but also perfectly sinergizes with the rest of your loadout.
The Power pimps your Toxin stacks and helps enable Hyper-Critical, while the Swap speed allows for constant adaptation to enemies: 
- See a Crusher er, switch to the Branx MkVI Needle Pistol hit 4 shots and switch back even before he manages one down swing.
- A group of Shotgunners spawn behind you? Quick switch and run safely into them/out of there.
- You get flooded by Ragers: Immediately go into a parry from shooting your Needler a second ago.

Now for the main act: The Catachan Mk VII "Devil's Claw" Sword 
This is what you'll be using 80% of the time, since your Branx MkVI Needle Pistol only needs a few seconds at a time to do it's thing.
The Catachan Mk VII "Devil's Claw" Sword with Hyper-Critical and Swift Endurance makes for a quick horde dispatcher. Throw your Stim into the mix and you can comfortably and safely take on anything other than groups of Crusher, Bulwark, or Scab Mauler. Light attacks and parries are everything you need and for the named groups you just need to add dodging and some toxin stacks to the mix which should be easy to do with your kit.
Stamina Curios give you attack speed and safer parries, add stim attack speed on top + Chem Enhanced + Float Like a Butterfly + Shred and you'll constantly be proccing Hyper-Critical. 
The Stamina Cost reduction from your Stim can set up a perfect combo of 1. Stimm Supply then parry anything while standing your ground, taking out/weakening key targets (doesn't matter who attacks, the parry always hits the enmie you aim for) and then Booby Trap goes off and clears the ground.

It can not be understated how defensively invulnerable the parry can make you, and with Regained Posture and profiecient dodging in between you can basically tank attacks unlimitedly.

Last but not least, bosses and Daemonhost are made easy with this build. Pump some Branx MkVI Needle Pistol shots into them (5 for max stacks and then afterwards 1-2 every other chance), prep against corruption with Stimm Supply and parry away (15%Stamina reduction) while sneaking in some hits when you can. 
Daemonhost are dead before your Stim runs out. 
For some bosses hit and run works just as well, but remember you can take the aggro well for a team in a tight room.