I don’t know which is “best”, but I’d probably go with money shot or the second one I list, simply because the best should be the best in every situation. In terms of my bias, Maya is my favorite character to play, though I’ve taken all of them to op8.
Most powerful skill: Money Shot… I don’t think anything else causes that much damage on a consistent easy to achieve basis. Yes, maybe melee zero can do more damage on a single hit or melee Krieg in a bloodsplosion or maybe with blood bath high enough, but those are trickier to setup than just grog harolding with a roughrider and a chaotic evil monk COM.
Most powerful “hidden” (not explicitly stated in the game) skill: Sal’s ability to merge weapon abilities from his left to right hand while gunzerking.
In addition to money shot, this makes him the raid boss king.
Most character changing skill: Release the Beast… Krieg is a different character with this skill as compared to without it, I don’t think anybody else’s capstone skill changes the char so greatly where you are playing them completely differently as a result.
Most game changing (solo): Phaselock… I know it’s just the action skill itself, but bar the biggest bosses, this, imho of course, is the biggest game changer. No other action skill stops an enemy from attacking immediately. Once you add in Ruin and Converge, you then add a slag singularity grenade to every phaselock as well, but even without those, the ability to just freeze an enemy solid in the middle of battle is pretty much as powerful as you can get.
Most game changing (co-op): Res: Nothing else can revive downed teammates from distance instantly. It’s easier to beat anything if you are alive