Planescape: Torment. It's my favourite game ever as well, so there's that.
As for why... well, let's begin with the flaws since there are many. The AI is rather poor, the combat flows like
bitumen through a funnel, there's so much stuff to read that I'm pretty sure you spend about as much time reading as you do actively playing the thing, there's very little voice acting and cutscenes, the pacing is slightly off and on and on and on it goes. But the story is great and immersive, the characters interesting, the setting unusual and the text fairly well written, and it's all so absorbing that I daren't touch the game too often because I know I wouldn't be able to stop anytime soon. Then someone decided the best game ever needed a few mods, and, amazingly, made it even better.