Pong sounds to me like kind of a copout. So excluding Pong, I would have to go with a game from out of left field: The Marriage
http://www.rodvik.com/rodgames/marriage.html
The reason for this is that it is the only pure example of gameplay as a mode of expression. Pretty much all other games which try to have some sort of expression behind it have game mechanics that at best prop up the expression it is trying for, and more and more, cinematic games like MW2 and Uncharted use non-gameplay elements to communicate a story or vision. In "The Marriage", outside of the absolute most bare bones, color based metaphors, the entirety of the vision and emotional impact is communicated through game mechanics and dynamics. That's not just innovative as a game or genre, that's pretty innovative as a way in which human beings perceive the world. It's innovative on an existential level, and I think that gives it my vote for most revolutionary game of all time.
