Planescape:  Torment is a great RPG, in the truest sense of the genre.  You play the Nameless One, a confused but apparently immortal (in the sense of if you die, you don't die for good) wanderer of the planes, whose very flesh tells the tale of his endless struggles.  You are turned loose into a world that bears only a passing and vague resemblance to any other D&D or RPG realm you may have encountered, and that is filled with endless mysteries and characters.  I've never played another game where you can spend an hour talking to the other NPCs in your party (let alone in the rest of the world), earning experience and knowledge for what you uncover about their lives, and never feel like you're bored.  There are plenty of fights but no sense of tedious hack-and-slash, and it's difficult to emphasize just how convoluted and intricate both the plot and the game systems really are.  As an example, at one point my Nameless character was festooned with power-granting tattoos, dual classed as a thief-mage, and clubbing his enemies with a severed limb from one of his own previous incarnations.  I mean, come on!