Planescape Torment is a cult classic and is considered by many to have the best story in any video game ever made.  I recently completed it, and I'm here to tell you that the hype is justified--this is an incredible game.  It instantly makes it into my all time top 5.  Planescape contains one of the most unique and interesting settings and the best writing and plot I have ever seen in a video game.  It's possible there are some adventure games that can hold a candle to it (I've heard the story in the game The Longest Journey is mind blowing), but I haven't played them.