Good question! Choosing just one is supremely difficult. Obviously the Lumiere brothers and their contemporaries are critically important in birthing the art of film, but in terms of the films' effect on the world by themselves, they're not that significant.
I can come up with a shortlist:
The Birth Of A Nation (1915) Dir. D.W. Griffith - The first 'epic', and also Griffith was crucial in pioneering editing techniques such as cross-cutting.
Battleship Potemkin (1925) Dir. Sergei Eisenstein - Early use of montage for empathy with character.
Citizen Kane (1941) Dir. Orson Welles - Precursor to all contemporary cinematography. And it's a good film too.
2001: A Space Odyssey (1968) Dir. Stanley Kubrick - Sci-fi and cinema in itself has never been the same since.
Star Wars (1977-83) Dir. George Lucas and co. - Really the first time the mythology of a film series has permeated everyday life, of which its fans follow it as faithfully as a religion.
But, it's a fairly impossible task to pick just one, as cinema has been influenced by a whole range of things, building up on what has gone before. Star Wars couldn't exist without 2001, which couldn't exist without Citizen Kane and so on and so forth. Film itself couldn't exist without theatre and theatrical philosophy dating as far back as Aristotle.