This is only slightly contentious, but the one film that really brought cinema out of the sideshow and demonstrated that it could be used to create a new kind of narrative-of-images in the silent era was DW Griffith's Birth of a Nation. It's the one film from that era for which there's a sense of before and after.

(Other contenders might include Eisenstein's Battleship Potemkin, which is used to this day to demonstrate to film students how "montage" can create a story, and as a result, is widely quoted by other filmmakers... probably most humorously by Terry Gilliam in one scene in Brazil.)

The problem with Birth of a Nation is that, as well as being hugely popular, (there was a special screening at the White House!), it was also a film about how great and heroic the Ku Klux Klan was in saving the noble South from the scourge of Reconstruction and the emancipated hordes of former slaves.

Quoting Roger Ebert (that second-to-last link there):

Griffith demonstrated to every filmmaker and moviegoer who followed him what a movie was, and what a movie could be. That this achievement was made in a film marred by racism should not be surprising. As a nation once able to reconcile democracy with slavery, America has a stain on its soul; to understand our history we must begin with the contradiction that the Founding Fathers believed all men (except black men) were created equal.

Griffith will probably never lose his place in the pantheon, but there will always be the blot of the later scenes of “Birth of a Nation.” It is a stark history lesson to realize that this film, for many years the most popular ever made, expressed widely-held and generally acceptable white views. ... "The Birth of a Nation" is not a bad film because it argues for evil. Like Riefenstahl’s “The Triumph of the Will,” it is a great film that argues for evil. To understand how it does so is to learn a great deal about film, and even something about evil.

David Bordwell & Kristin Thompson (as mentioned by Ebert further down that essay) have catalogued the innovations Griffith used to create his story - things like cross-cutting, establishing shots, POV shots, irising and matting, and attention to everything from costume to setting to, er, makeup. (In a film in which half the actors are in blackface, saying there's attention paid to makeup is a little galling, but yes, it's true.)

EDIT TO ADD: Filmsite.org has a pretty good write-up covering both the innovations and the... well, Ebert calls it evil. I'll go with that. The evil.

Unfortunately that would probably be DW Griffith's "Birth of a Nation".
Viewed today as one of the most racist films of all time it was also hailed as groundbreaking, historically important and one of the most influential movies of all time.

At the time President Woodrow Wilson said: "It is like writing history with Lightning. And my only regret is that it is all so terribly true."

James Agee, one of the mid 20th centuries leading film critics said:

"He achieved what no other known man has achieved. To watch his work is like being witness to the beginning of melody, or the first conscious use of the lever or the wheel; the emergence, coordination and first eloquence of language; the birth of an art: and to realize that this is all the work of one man."

i found it very hard to watch. It is totally unabashed in its approach and so terrifically out there when it comes to its political point of view. But many of the innovative storytelling and cinematography techniques that it pioneered are fundamental and taken for granted by today's film makers.