gyokzoli: Probably either Planescape: Torment or Icewind Dale.
Fenixp: Both Icewind Dale and PST will be the same every time you replay them. Sure, you can change your party and completely overhaul your tactics in IWD, and make different decisions and get a comletely different character in PST, but the progress will always be generally the same.
I would actually go with Icewind Dale as well -- the custom party creation and wide variety of available races, classes (including multiclass / dual class options and specialist Mages) and alignments, as well as the randomized loot tables and various difficulty options (including Heart of Fury mode) provides incredible replayability.
There is also a surprising amount of class- and race-specific dialog options in the game which sometimes provide unique solutions to encounters (a Druid, for example, can give Ghereg, the ogre-with-a-headache in Kuldahar Pass, the recipe for a cure made from natural ingredients).
Edit: I agree that NWN Diamond is an incredible package that would provide near endless amounts of content, and I would have picked it, but I can't stand the graphics style... From the clunky tile-based structure of the areas to the extremely blocky characters -- I thought it looked bad back when it was released, and I think it looks worse now. I'm normally not someone who's put off by a game's graphics and I play a lot of old games, but NWN... Man. I'm telling ya...
Fingers crossed that GOG.com will eventually get their hands on NWN2 Platinum :)