It's probably not controversial to say that the Dungeons & Dragons game from 1974 was the most important and influential game in the whole genealogy of all RPGs. Every game that followed was either an attempt to make a game that improves on D&D or make a game that is different from D&D. They were either a direct response to D&D, or expanded on a game system that was.
The real question is, what other games can be considered hugely influential on how people design game systems today?
I've been thinking recently about what huge an impact Apocalypse World had. I think it's never actually been very popular as a game that is being played and I've seen plenty of people having major issues with that. It's probably become overshadowed in those regard by ots descendant Blades in the Dark. But this whole branch of the RPG genealogy is called Powered by the Apocalypse for a reason, and I do still see it referenced in passing when it comes to discuasions about developments in game design. It certainly was the mosy eye-opening game I ever read, opening up completely new perspectives of what RPGs could be.