Good lord. If there's one game out there that's art, it's Planescape: Torment. And it's not there. The pure depth and richness of this game, and the questions it asks about human nature and evil make it every bit the game equivalent of any literary work.
Hm. Neither is Continiuum, (Alpha Waves) which is both historically significant and the equivalent of a tone poem.
http://www.youtube.com/watch?v=bF_a6qMeWP8 (Alpha Waves)