I'm in a toss up between cell and buu. Cell is obviously the most skilled fighter, with all the skills and techniques of all the characters in the show, and a superb regeneration ability. But Buu's regeneration makes him almost invincible. He is nowhere near cell in terms of intelligence or skill, but the fact that he bascilly can't die unles he is pulverized into absolute nothingness gives him an edge.
But now that I think about it, cell needs pretty much the exact same thing done to him, just when he gets blown to bits he regnerates slower.
I dunno, its a toss up between those two unless you get rid of there regneration powers, then cell would win hands down.