Hmm... Good question... I rather like all of them, actually. The show itself has had its ups and downs, but TUF has provided a lot of good fighters for the UFC.
My favorites of the winners:
1. Forrest Griffin. Seems like a wacky guy, in and out of the Octagon, and despite the clocking by Jardine I still think he can be a contender.
2. Matt Serra. "Hammafistelbow!"

Don't tell him I said so, but I kind of hope he loses to Hughes and goes back to 155. I'd love to see him take on Sean Sherk and B.J. Penn and all the other guys.
3. Rashad Evans. He exceeds my expectations literally with every fight.
4. Joe Stevenson. Gained a lot of points for folding up Melvin "Blowhard" Guillard like a tent. Seems like a good guy and a potential contender.
5. Michael Bisping. Cool accent, good nickname, good writer and, oh yeah, he can fight a little too.
6. Diego Sanchez. I was horribly disappointed with his effort against Koscheck, but I think he'll bounce back.
7. Nate Diaz. I normally like Parisyan, but I thought he was being kind of a jackass.
8. Kendall Grove. I don't have a strong opinion on him yet. He's proven to be a better fighter than I expected, but I don't know if he's a potential contender. I am looking forward to a rematch with Ed Herman, though.
9. Travis Lutter. Like Grove, I don't dislike him. Just kind of "meh", I guess.