Metropolis is probably the all-time classic. I would also suggest Birth of a Nation, even though it is blatantly racist, because it is one of the most memorable works of the period. Intolerance was the next film that D.W. Griffith shot as a result of the charges of racism filed against him even then, and that movie had the largest sets to date (he rebuilt Babylon as a Hollywood set that was too big to tear down all at once, so it remained for years and years). Nosferatu and The Cabinet of Doctor Caligari are German classics. Pandora's Box is a great example of a very risqué movie from the period. Foolish Wives by the great Eric von Stroheim is another good bet. On the Russian side, Aelita (??????) and Battleship Potemkin (?????????? ????????) are good. The original version of Ben-Hur is also good.