It's so tough to single out just "one" game over so many damn years but I always seem to come back to Goldeneye on the N64, I don't know what it is, for it's time, my personal enjoyment and the countless-countless-countless hours I've put into multiplayer and lugging that game and system around to my friends house on the weekends to stay up all night playing multiplayer yelling and screaming while his mom is yelling at us in the basement to get to sleep is an era I'll never forget in my gaming history. Man, I played that single game for YEARS and never became completely bored with it. Nostalgia aside, that game really left a mark on me.
I still remember when I moved out for the first time and got an apartment with my girlfriend and another buddy of mine. We didn't even have a couch and we had an old TV up on top of a milk crate and I remember unpacking my N64 (which I haven't played in ages at the time) started going through the games and just yelled out "We're playing Goldeneye tonight, dammit!" sure enough, house warming party of just shots/beers Goldeneye and Turok all night, I believe I actually threw up once we hit Star Fox 64 though.. the spinning and panning was just too much for me to handle. lol. Good times.