After watching highlights from their last fight for the past few weeks, I have to say BJ Penn. He was a very immature fighter the first time, but if you saw his fight against Sean Sherk, you can see how he really upped his training.
BJ's best (only) chance is to have an explosive opening and KO GSP quickly. GSP's wrestling and takedown defense are enough to nullify BJ's chances of showcasing his Brazilian jujitsu. Plus - GSP has vicious kicks and craaazy endurance/workout ethic (if you watch ufc prime-time, you know what i mean). Not to mention GSP is a natural (and huge) 170 pounder, BJ is packing some extra weight to get into the fight.
Believe me, I'm saying this as a real Penn fan, he is a psycho and his flexibility, and jujitsu are unmatched, I just happen to think GSP can do enough to keep him subdued. Either way, it's gonna be a great fight.