Planescape: Torment
Planescape: Torment has held my interest longer than BG 1 & 2 and IWD 1. Which is surprising when some dialog trees are longer than chapters in books and, in general, video game's stories are shameful. Very compelling.
If anyone hasn't played it I suggest you do. And I suggest you stick with it for a few hours then it'll really wrap you up.
Please no spoilers. I'm only at the beginning of the 100+ hour adventure.