It's pretty hard to say actually...when I first entered this topic I was going to say Liddell by a long shot, but looking at his record Liddell called it quits at a good time. Lost to Rashad, lost to Shogun, then lost to Franklin all by KO.
Thing about Penn is that he's not done, he just trains like shit and fights at WW instead of LW. The dude is small, and has low muscle mass; the fact that he fights the way he does is reason enough to believe in the guy. It's just sad to see the guy refuse to re-invent himself, he really could be one of the best again, but he just doesn't care anymore. If he really wanted to prove something he would get a ripped, athletic body, and train at a respectable camp with people who will push him and try to break him.