pretty hard question, i would distinguish between multiplayer and singleplayer. As far as multiplayer experience is concerned my choice is clear, definitely Duke Nukem 3d. Still know all levels and secrets by heart from the thousands of coop runs and in deathmatch trip bombs, lasermines, jetpacks and holodukes were just pure genius.
In singleplayer no game has stolen more of my time than the civilization series, so thats my pick there.