Hellboy has the power to regenerate, strength that would kill a normal man, the Right Hand of Doom, and the distinction of being a demon (Son of the Fallen One). I gotta give the edge to him.
Well, considering Hellboy's personality, the first word to be throw around would probably be "asshole."
We know how much Hancock loves when you call him an asshole.
On characters terms, I'm gonna go with Hancock, something tells me he's stronger than Hellboy, plus, he's immortal, as long as certain person is not around.
On movie terms, I'm going with Hancock too, it's not perfect, but at least is entertaining, unlike Hellboy II, which is boring to no end.