Planescape: Torment, probably. Tried to go back and play it, but it's just too ugly. I missed it at release because I was young and naive, and to my knowledge there wasn't a Mac version, which flew in the face of the fact the Mac was the dominant gaming OS of the time. Missed Baldur's Gate too, but thankfully it's not my style. Sword & Sorcery is the weakest setting of all. Unfortunately, that's where most good RPGs are. But I can afford to miss 1.
Though I am not regretful or anything, i was playing Warcraft, Warcraft II, Diablo, Myth, Marathon, Prime Target, Abuse, Escape Velocity and many many super sexy games at the time, some of which most if not 99.9% of Windows gamers have never seen, so all in all I got the better deal.