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?
Warhammer and MechWarrior for miniature combat for me. They also helped me understand how to be a DM for pen and paper games on what was possible and what was just pure fantasy.