Fallout 3
aka VAN BUREN the REAL Fallout 3!!!

I've only played Fallout 1 and 3 and for me Fallout 1 is the better game. There is a real charm to the graphics (almost like a 3D Flashback, I think) and it's just a refreshingly fun RPG. Great game!

Though it has its flaws, Fallout 3 is the ultimate rendition of the series for me.

Look at the downloads section of nma-fallout.com, there's a set of engine tweaks for Fallout 2 that do various things (mouse wheel support, fix some broken formulas, etc)
One of the things it does is allow you to disable the 13 year time limit (or set it to zero so the game ends before it even starts!)

Fallout 2 wins for me, bigger and it has better control of NPC's, not to mention the very nice ability to push people away from doors and such, Fallout has a better story though, so it's a very close match between the two.
#3 I haven't played and I don't think I ever will, not my cup of tea or my kind of game, I found Morrowind farely fun at first, played it a couple of weeks 'til it bored me, felt like all I was doing was travelling. (pretty much the same feeling I got from Gothic 3, seems like #3 in a series is something I should avoid ;))