My vote would go to some Metroid game or another, or perhaps Final Fantasy VI. Though if you're talking Side-Scroller, definetely Metroid something. Since it's not on there, my vote must go to Tetris. Not because of its great qualities as a game, but because it's practically the first game ever that spawned all kinds of other "emulations" of it that never live up to it, and it's just so darn addicting (at least for me).