As a kid? I know mine is Star Tropics on the original Nintendo. Then the original Nintendo game "Anticipation" where you had to name what the pencil was drawing an outline of before time ran out and it connected the dots.
I also remember the very first golden cartridge Zelda and Zelda 2:Link's Adventure and playing the hell out of them, which is probably what spawned my love of anything RPG (MMO or otherwise).
So, what game did you first play? And why that specific game? Be sure to leave some info and not just "This Game" and "IT was fun or it was on hand." If you're a true gamer, then you know your own reasons.