Tankiest Fire Support (+10% T/s for the team)
 Skitarii Build - Warhammer 40k: Darktide 

Toughness: 207 | HP: 150

Total Toughness Regeneration per second: 44 T/s

Resurgence Aura: +3.75 T/s | (+7.125 T/s for players with 3x(+30%) Toughness Regeneration Speed on curios. Not worth.)

Power Redistribution Uplink: 207*0.025 => +5.175 T/s

Auto-Repair Doctrines: 207*0.04 => +8.28 T/s

Entropic Transfer: 207*0.03 => +6.21 T/s

Binary Ballistics Protocol: 207*0.05 => +10.35 T/s

Superior Defence Engrams: 207*0.05 => +10.35 T/s

Team gets 44*0.25 => 11 T/s from Power Overflow +3.75 T/s from Resurgence Aura
Total T/s for the team = +14.75/s 

So at best, this provides the equivalent of 10% toughness per second to a teammate with only 150 Toughness, and lower if they have more, only 7.5% T/s for someone with 200 Toughness. It is not bad, it's about 1 talent point worth of Toughness regen for each teammate, IF you can maintain full toughness. 

For example in the event of a 4 man Poxburster explosion, everyone is at 0 Toughness, everyone will do what they can to recover. You will recover to full in about 4 seconds, and only after that your team will start getting your excess 12 T/s. But even if the others recover half as quickly as you, they'd already be at 50% in 4s, so they have a T/s rate of 12.5% already, and now you boost it up to about 20% for the rest of the way, helping them recover that remaining toughness in 2.5s instead of 4s. All in all this is about a 30% faster toughness recovery time for your team, at best. Considerably worse if your team is already good at recovering toughness, and nothing at all if you are always worse at that than they are (but that is unlikely with almost 50 T/s in combat).

That in itself is not so bad, Ogryn's Stay Close! aura does about the same at 20% (or at least it would if it worked on all Toughness).
However, this provides even more than that if you can maintain full toughness most of the time. Because it provides constant Toughness regen to your entire team, something that most classes do not have in combat unless they can actively attack, it protects their health pool from chip damage. For example when they are repeatedly smashed by a Mutant, the constant Toughness regen you provide will recover fast enough to prevent any health damage at all, which they would otherwise take. In that example it's not much, maybe 15-20 HP lost at most, but still that's just one example. It obviously helps a lot with little shooters peppering your Ogryn while he is trying to shoot back and so on.

So is Power Overflow worth it? 
It sure is worth the talent point for your team at least, by you spending 1 of your points, you effectively give the others 1 talent point worth of toughness regen, that's a 1 to 3 conversion rate. Especially good if you are a full Skitarii team, even if they have mostly different builds, if they have Power Overflow and max Toughness on curios, they will each add about the same T/s, as you already provide the Resurgence Aura anyway.

But if you use Inspiring Barrage it's even better! 
It stacks up a lot faster if you have 39 ammo in the mag instead of 40. That's why we need 70% ammo stat on the gun instead of 80%. 
With this blessing, over the course of unloading a full mag, which takes 15s, you will recover 2.2 times your entire toughness bar. 
That's 207*2.2=455 T in total, and 455/15= 30.33 T/s, your team also gets 25% of that => 7.58 T/s for the entire team. 
But that's just an average, it takes 5-6s to ramp up and after that you get 9s for the rest of the mag, at full stacks you are actually recovering even more T/s for the team:
207*0.2= 41.4 T/s for yourself => 41.4*0.25= 10.35 T/s for the team

Highest total T/s possible for the team:  25 T/s
Now that's as much as a 200 Toughness Ogryn with Steady Grip can recover, but it's for the entire team, and even better if your team has lower toughness or struggle to recover toughness while not dealing damage such as when dodging gunners or doing some objective. 

Now to quickly make a point for the other talents which improve your own survival:

Galvanized Coating is just so nice, and a small price to pay. By itself it allows you to survive two Poxbursters.

Overcharge Transfer Lattice is much better than Critical Power Overload, because unlike the latter, it actually stuns the enemies, and even though you do take the full dmg from the hit which procs it, because they are all stunned you get to recover that toughness before they attack again preventing you from taking health damage for a few seconds even while trapped.

And yeah of course we take Power Overload because this is a support build, it's also cheaper than any alternatives and provides more damage reduction than the other keystones too.

For being so tanky you still easily shred everything with your Branx Mk IV Arc Rifle, and infinite cleave push attack when the Branx Mk III Arc Maul is empowered. Chordclaw Strike can one shot Crushers and destroy Monsters and Captains.