It’s Mario Bro’s or Mario Bro’s 3. Nothing else even comes close to their sales PLUS impact on the industry. No matter how much feel good nostalgia tells you another game is

I played Super Mario Bros. 3 in 1989 so I'm totally justified in voting for it. It's one of the best 2D platformers ever made, even to this day.

Phantasy Star, my first ever RPG, at the very end of that decade. I remember being blown away by the 3D subterranean labyrinths, the fact I could move in those in the first person, totally blew my mind back in those days, even if those were very rudimentary and maybe 10 frames per second at the most.

The complexity of the story was something I had never seen before, the RPG genre, totally new to me. In fact that game had such an impact on me that the next game I tried after that (Ultima 4) completely disappointed me as it had none of the game mechanics of Phantasy Star.

Totally agree, I still think the only 2D platformers that can challenge it quality-wise are Mario World, Yoshi's Island, and the Donkey Kong Country games.

The Legend of Zelda, of course.