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?