I never saw Greed, but the story behind it is amazing. Talk about dedication to the source material...
On topic, my favorite would have to be The General. Even after all these years, the humor still stands up. Plus some of those stunts are remarkable, especially knowing it was really Buster Keaton on a moving train, putting his life in danger for a joke. I mean, dang.
On a side note, I really disliked Un Chien Andalou and Battleship Potemkin. I appreciate them and am glad I watched them so I can understand their effect on modern filmmaking, but they are awful to watch. The eye slicing part. *shudder* My distaste for Potemkin mainly comes from having to watch the Odessa Steps sequence at least 10 times during the course of my college education. And then watch a dozen movies that reference the film.