ranked by most personal play time:
T1
T:V
T:A
Legions
T2
I was a beta tester for T2 but it never really felt right to me. I quit before Classic came out along with the mass migration of T1 players who returned to T1 because almost everyone agreed T1 was just way better. I think that's when I joined =ACE= after cyanide continued playing T2 with Area51 (for those reading jerry's story thread).
By the time the T1 revival died down again, T:V was out and I spent a solid couple years with T:V. I shamelessly admit that I loved T:V. Many of the best T1 players made the switch so the community largely accepted the game and there were always familiar faces playing on the 5150 servers. Three things killed that game: a botched release date, ignorant corporate fuckheads pulling the financial strings (i.e. no patches), and the grappler. You either loved the grappler or you hated it and thought it bastardized Tribes. I loved the grappler. It added a new challenge to master and introduced some new elements to the game. After you got the hang of it, the shit you could do with the grappler was so damn fun. That being said, I can also understand why some people didn't like how it changed the game.
So much of my youth was spent on Tribes. Wish I had spent that time practicing guitar or doing something else more productive, but I'd be lying if I said I don't look back at the Tribes days as some of my favorite memories.