Unfortunately, I haven't had the chance to watch either season 4 or season 5 of TUF but given how dissapointing season 4 was reputed to be, I'm going to have to tip my hat toward season 1.
It was the first time this was happening in the UFC and it was all 'new' to an extent, ya know?
Not to mention, the whole Leben-smash crap was pretty damn funny.
