It's got to be Yamamoto surely. IIRC Death is the most powerful chaaracter in Soul Eater but Yamamoto has just become something else since he unleashed his bankai. Even Juha said that if he didn't have his quincy defences up he'd have already been burnt to a crisp just because of Yamamoto's bankai being unleashed. It's so powerful that it's threatening to destroy SS just by being there.
Given that he's also got a pretty much impenetrable reiatsu defence and the fact that we still haven't seen what his North (and most likely powerful) form can do I think Yamamoto should win this one without too much trouble.