Hands Down: Baldur's Gate.
Best I ever played during that time, and best I've played since. Nothing comes close.
I'm hoping Dragon Age: Origins will be the next winner.
EDIT:
New Troll said:
Baldur's Gate and it's clones where nice, even if very repetitive, because of thier immersion (at the time.) Problem is, Morrowind came out and showed the world what true immersion meant. True freedom, both to explore and role-play. So that basicly just left Black Isle with thier glitches and bugs. Still awesome games in thier time.
I would say that, by
far, Baldur's Gate was far far more immersive then Morrowind.
There was little to no actual role-playing in Morrowind! You didn't even have conversation choices (Very rarely)! The only thing really great about Morrowind, by comparison, was the graphics.
But the story, the NPCs, the dialogue, the quests, and pretty much every other part of the part of the game was completely and totally lacking compared to Baldur's Gate, or even Icewind Dale! And I remember very few glitches or bugs in any Infinity Engine Game, and heaps and piles in Morrowind (Continuous crashes, errors, and the like).
Don't get me wrong, I loved Morrowind! I got it the day it came out, and had a blast running all around that crazy island. But I still, to this day, play Baldur's Gate 1 and 2, and both 'Icewind Dale's, over and over. I couldn't play Morrowind again if I tried.