Planescape: Torment. There is no better. Plot. Theme. Character Development. Everything.
Second place goes to Dragon Age: Origins, because it truely is the most emersive rpg made. I wish someone would remake Planescape: Torment with the Dragon Age: Origins gaming engine. That. Would. Be. Awesome!
Third place goes to KOTOR. If I were ever knocked with amnesia, that would be the first rpg I'd play again, simply to be completely BLOWN away by the climax all over again. (however, with the climax known, the game does lose a lot of its replayability.)