I haven't voted yet. I'm torn between Atari 260, NES, and PSX.
Atari 2600 was the system that proved that a significant market existed for video games in the home.
NES was the system that made gaming ubiquitous.
PSX ushered in 3D graphics, and was the first significant console released by non-gaming focus company. Without the success of PSX, I doubt that we would have MS in the industry now. Without Sony and MS, the gaming world would be a really different place.