What's the greatest 2D game ever made in your view, and why?

[firebush03]

KLXVER said:

Thats a bit vague. I mean its like asking what the best 3D game ever made is. It would just be your favorite game.

I think it’s a good question. Basically, it’s just as valid as asking “What is the greatest game ever made?” but with the restriction to 2D. So no Ocarina of Time or Resident Evil 4.