My vote goes to Vampire:Bloodlines. Even without the player created patch it was amazing(despite the publisher pushing it out before it was ready), immersive, various ways to complete the many quests, etc. Amazing. The soundtrack was excellent too. And the voice acting. A lot of replayability, I rarely play an RPG more than twice let alone about 8-10 times. Ok i'll stop.

Some honorable mentions:
Baldger's Gate II
Fallout II
Fallout New Vegas(Fallout III was irritating due to using a few voice actors, bad ones at that, for many characters.)
Arcanum(I could never get interested enough to finish it, but a lot of it was unique and interesting, particular since it was before the steam punk craze)