If we're talking non-plot-armor, non-bad-writing pure power, I'd say The Beyonder [http://en.wikipedia.org/wiki/The_Beyonder_(comics)]. Dude was basically omnipotent, at least when he first appeared. He was actually capable of destroying death.
(Okay, I know it was asking for superHEROES, but this guy was more Chaotic Neutral than evil.)
(Okay, I know it was asking for superHEROES, but this guy was more Chaotic Neutral than evil.)