mine is the donkey kong game from nitendo 64
Planescape: Torment - One of the best story in a video game ever.
TES 3: Morrowind - My first ever TES game, my older brother got the GOTY version on the original Xbox, at first I absolutely hated it, but after several hours and several characters later, it just clicked and I played for hours.
System Shock 2 - A Cyber-punk FPS/RPG Survival-Horror. All my favorite genres melded into one brilliant game.