This one has the widest potential pool of choices since current generation consoles have only been out so long, while PC gaming is decades old. How far back is too far? Deus Ex? Starcraft? Fallout? Zork? Plus there are still great games coming out recently, like Crysis and Minecraft (ironically polar opposites in terms of graphics).
I'd have to go with System Shock 2. It was immersive, creepy, and fun. I'm glad we're getting a new Bioshock game, but when is EA going to let go of the license so we can get a sequel for System Shock! Screw GlaDOS, Shodan is the queen of scary AI.
