Well, pretty much everything in Ultima was either innovated or popularized there. It came out in 1980, there really wasn't a lot before it with any kind of complexity.
But each game from the Ultima series was additive, and Ultima also pulled from Akalabeth, so it's hard to pick a specific game to be "most influential." Is it Ultima I because it started the series that largely standardized CRPGs? Or is it Akalabeth because its success led to Ultima?
And that's why I had a problem with this from the start as Ultima 1 really wasn't that ground breaking compared to others but by 4 you have an unspoken karmic system that tracks level advancement that blew my mind as a kid once I realized that was a thing.
Ultima absolutely pushed significant boundaries but I have a hard time saying it was more influential than tetris.