Mario is pretty valid, but I'd go with DOOM:
It was enough of a sensation for Bill Gates to get in a trench-coat for a cringe ad because it would move more copies of Windows itself.
It established such basic conventions in its genre in both design + tech that others after for the next decade were called Doom-clones.
It built core aspects of online multiplayer, establishing netcode conventions and game modes like deathmatch.
Mostly though, it was the first game to create robust tools to make modded content that lives on to this day, whole new games were made out of those tools, and it created more NEW game designers than any other single game can possibly influence now.