System: Super Nintendo
The Legacy: In 1991, the Super Nintendo took everything we knew about video games, and made it slightly more complicated: Not just two buttons on the face of the controller, but four. "A" and "B" for the original buttons, and "X" and "Y", named after axis alignment planes. Like the Game Boy, the buttons were offset diagonally. The four aligned in a diamond formation that would eventually influence the controllers of systems that were barely a glimmer in Sony or Microsoft's eyes, yet (the PlayStation and Xbox controllers, of course), which is to say nothing of the shoulder buttons that changing gaming forever, as well.
Finally, the controller's most important innovation? Something Sega had already beaten them to with the Genesis: A rounded base, meant to align more naturally with the way gamers actually held controllers (as opposed to the hard, boxy ways of NES controllers that jabbed and bruised many a hand).