The Ultimate Anchor ⚓️
 Arbites Build - Warhammer 40k: Darktide 

Curio setup:
One 20% HP curio ensures you can survive 4 big hits instead of just 3 when combined with True Grit . 

Stacking 3 of the same Damage Resistance is way better than only taking 2.

One DR stack of 20% translates to a 0.8 multiplier on incoming damage from those enemies. Which translates to 25% increased eHP against those enemies, not just 20%, because this is how eHP is calculated where 100 is your base HP: 100 / 0.8 = 125 eHP

Two stacks work like this: 0.8 * 0.8 = 0.64
So instead of reducing damage by 40%, it's reducing it by 36%, sure that's a little worse than it could be if it were additive. However, stacking them like this will still give you more eHP than expected. So at first with just one stack, our eHP was increased by 25%, with two stacks, at 36% damage reduction instead of 40% you might expect that our new eHP increase is less than 50%, but it's actually better than that: 100 / 0.64 = 156.25 => That is 31.25 extra eHP compared to the one stack version, and 31.25 is 25% greater than 25, the previous increase in eHP.

Now finally with 3 stacks: 0.64 * 0.8 = 0.512
That is about a 49% damage reduction, but it's almost double the eHP compared to base HP: 100 / 0.512 = ~195 eHP

So its ALWAYS worth stacking the damage reduction perks on the curios as much as possible rather than getting one stack of each.
Especially because these damage reductions are further multiplied by any other damage reductions you get on the talent tree and so on.

One slot is always for the triple revive speed, which leaves us two enemy types we can reduce damage from.
Obviously we want gunner res in general, but also when we have to revive someone while getting shot, so that we have TWICE as long before we get interrupted from losing all toughness. 
The one other slot I use for Bomber res, saves a point from having to take Rebreather , allows you to sit TWICE as long inside fire or tox bombs to revive or get saved from a net without downing. I believe it also still reduces dmg taken from gas generated by the tox gas maps, not just from tox bombers.