For me personally it'd be Lex Luthor. His appeal lies in the moral grayness of his character. Could he really be good if Superman did not exist? Or is Luthor just saying that to appease his own ego? There have been times when he did save the world, but there was always the fact that he benefited from those stints as a 'superhero.'
Though I've got no idea what he can get out of being Superlex.