I'm going to go with GSP based on his competition. Even though he tends to dominate George always seems to give the fans at least two solid rounds of fighting. But anyone of the truly elite fighters are a pleasure to watch just based on the skill they bring to the table.
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.