Super Mario 64 - Even though I had the SNES at a really young age, this was the first proper game. Just pure fun as a kid.
Super Smash Bros (Original) - Was the game my cousins used to play with each other. Looking back I have come to appreciate the simplicity of it compared to its sequels. Granted its hard to get back into since Ultimate is the definitive casual iteration and melee is the definitive competitive iteration.
Star Wars Knights of the Old Republic - This is the game that got me an Xbox. To me, it was just mind-blowing and to me, it is the definitive Star Wars game without question. Storytelling, characters, plot were all top-notch. The light side, dark side aspect was perfectly done. I really wish they would do another Star Wars RPG.