TUF 1 was by far my favorite. In my opinion, it's the only season where the majority of the characters were interesting AND could fight. I think the follow up seasons had a bunch of guys who weren't that good. Of course each season had some good fighters, but I felt that in TUF 1 almost all of those guys could kick butt (Hence almost all of them are now fighting as regulars in the UFC).
TUF 2 was very bland. TUF 4 sucked huge donkey nuts, but TUF 3 was pretty good. I'm really hoping 5 is awesome.