My personal favorite old movies are John Ford's The Long Gray Line and Buster Keaton's Sherlock Jr. John Ford is a classic old-movie director (Grapes of Wrath, The Man Who Shot Liberty Valence, The Searchers), but The Long Gray Line is a seriously underrated one from him!
I've been watching some great old movies lately. I'm focusing on stuff from the 20's, so I've seen Erich von Stroheim's Greed, Foolish Wives and The Wedding March, and Buster Keaton's Seven Chances. Great stuff!