Definitely Super Mario Land on my very first game system ever, that wonderful gray brick, the Gameboy.
Actually, I'm sure I played Tetris first, but my 5-year old mind didn't immediately comprehend the objective of the game...so, Super Mario Land it was. Certainly got my share of time into it, I think it took me months to finally get all the way through that one.
That Gameboy kept me company all the way through grade-school and beyond. I was still playing that thing occasionally in middle school.