Super Mario Bros - Pure classic, pure platformer, and was one of the first titles that brought home console gameing back from the abyss.
Legend of Zelda: Ocarina of Time - Classic! RPG elements, puzzle solving, easy to learn, but still challenging. This one game included so many elements that we take for granted today.
Tie: Halo 3 and Bioshock:
Halo 3 - A good starting place for any FPS fan. Good design, good story, and one of the best multiplayers around.
Bioshock - Pretty much better then Halo in everyway, story, level design, controls, but no multiplayer.