First Son is comfortably more powerful than Baras to me, but with Syo wrestling him for control I can see your point. Though Syo won't be healing any of his adversaries, this time.
Thanaton's pretty powerful but I've always depicted the weakened Voice as being more powerful. The only one who could properly hold a candle would be the First Son, but with Syo wrestling him mentally, I think Vitiate's Voice could pull it off.
Though this is a free-for-all, so it's hard to tell who would fight who, exactly. I'd expect Vitiate to attack Baras in revenge, whilst the First Son is probably still loyal to Vitiate and would likely fight Thanaton.
Voice probably takes this. I'm curious as to why everyone is saying Thanaton is at the bottom of this. I've always viewed him as stronger (in terms of Baras in final fight and Thanaton final fight). He did say this was final boss fights, IMO Thanaton in his final fight before death > Baras in his final battle before death. Then it's probably First Son under Thanaton for reasons of Syo fighting back mentally.
You have scoured the galaxy and bound enough spirits to challenge Darth Thanaton.
You found Thanaton in his meditation chamber on Dromond Kaas and confronted him. Speak to the defeated Darth Thanaton.
Seems like a mistake? Nox doesn't engage Thanaton at all. Unless its like he's defeated because his apprentice died or something non-literal.