The NES, Xbox or Dreamcast. My trouble with deciding is this:
Nes: Kickstarted the video game industry, have us the standard for control pads, revolutionized and even invented game genres and was highly succesful.
Dreamcast: Revolutionized online gaming with Phantasy star Online. Also, came with internet adapter, and even a second screen on the controller with a VMU. Also, experimented with new and weird game genres.
Xbox; Having a harddrive changed gaming forever. Also, made online popular and a standard for gaming going forward. Through Bungie, made fps mainstream within console gaming.