One of the best RPGs have played IN MY LIFE. Period.
Planescape was something entirely different. Instead of the usual something-is-destroying-the-world storyline where the protagonist needs to go on a journey and gather evidence and eventually defeat the source of all evil, Planescape is a journey of self discovery. I played this game after playing all the latest ones so I found that the game lacked a few things, but comparing Planescape to games that came 8 years after it did is hideously unfair. The storyline and the characters are very memorable, though I must say that the game lacks as far as items such as weapons or armor is concerned... the plethora of items in Dungeon Siege only speaks against Planescape.
No matter who you are or what you play, I STRONGLY recommend playing Planescape:Torment. Not only will it grow on you, but you will find yourself playing it over and over again. I only hope other games can achieve the kind of immersive environment that Planescape has...