The original Civilization. Also the original Railroad Tycoon. I played those for years.
In addition, X-Wing, System Shock, Fallout, Planescape: Torment, and Rogue Spear/Rainbow Six.
And lest we forget the real golden oldies: Zorks I, II, and III, Hitchhiker's Guide to the Galaxy, and various other Infocom classics.
-- Todd/Leviathn