going to have to go with Tito, GSP and Penn. Based upon skills, attitudes and entertainment value.
I know people are jump on this saying Tito is a punk who doesn't respect his opponents but I think his post match antics are just that, antics. I believe you can measure Tito's respect for his opponents by looking at the rediculous training he does for his fights.
Hating on Anderson Silva before it was the cool thing to do.
The idea of a "true number one contender" is the stupidiest MMA idea to be perpetuated by fans and media, EVER.