Either:
Donkey Kong (arcade) - first platformer ever, introduced two legendary mascots, put Nintendo on the map, or...
Super Mario Bros. - one of the first side-scrollers and substantially influenced gaming going forward
I could also make a case for The Legend of Zelda with how it blended so many elements together for such a new experience: Arcade-style gameplay mixed with open-ended design and intelligent item use for progression. Also, saving on cartridge (disk cards in Japan).







