difficult to say.
Morrowind is great because of its brilliant plug ins but is quite slow as a game (but still gigantic) while Kotor is good but small.
Never finished playing Planescape Torment but I loved the part I did play (but I haven't played it enough to cast a vote for it)
Just started playing Fallout 2... pity my computer crashes every few minutes
The game doesn't seem very exciting yet since the controls are really awkward. But I guess it still needs to grow on me.
I choose Knights of the old republic for its wonderful sidequests, cinematic storyline and amazing deep characters.