It's a tie for me:
The NES for how it affected the industry: Established the 3rd party business model, made D-pads standard over 8-way joysticks, and of course, how its library helped to shape gaming going forward. Not to mention how it resuscitated the home gaming market in the US after the crash of '83.
The SNES for so many stellar titles in its well-rounded library of games and how gracefully it aged.
Both are my two favorite systems of all time.