Easy: Battleship Potemkin, by Sergei Eisenstein. When watching this pioneering exercise in montage, one doesn't even notice the fact that it's a silent film. Brilliant--especially that celebrated "Odessa Steps" scene.
Mine is a close tie between Victor Sjostrom's The Wind and F.W. Murnau's Sunrise, followed by Carl Theodor Dreyer's The Passion of Joan of Arc and Yasujiro Ozu's I Was Born But....
Well, two of my favorites were already mentioned, "Napoleon" and "Potemkin." Actually, "Napoleon" was the first that came to my mind.
So, I think I'll pick "The Big Parade" -- it's a terrific, entertaining film that doesn't often get mentioned, and it stars John Gilbert, who was a first tier star at the time, although he also doesn't get mentioned a lot anymore. Saw this film on the big screen with a live orchestra and an in-person introduction by John Gilbert's daughter, who penned her father's biography. Needless to say, it was easy to understand why this was a huge hit in the late 1920s.
Runnersup: Buster Keaton's "The General," "Wings," and "Ben Hur, a Tale of the Christ." All these films prove that lack of sound is no detriment to artistry or entertainment.
Dreyer's The Passion of Joan of Arc
Others would include:
The Wind, Nosferatu, The Last Laugh, Steamboat Bill Jr., Sherlock Jr., The Gold Rush and Broken Blossoms.