Often ignored, the silent works of the 1920's (and before!) are as important to cinema as anything else. It's generations ago, sure, but many of our more contemporary concerns about government, class, and religion are still addressed, possibly more directly so. The lack of the term "political correctness" at the time is a blessing in this regard as films could afford to be straightforward rather than dancing around the issues. Are they as entertaining as our modern cinematic spectacles? No, of course not, but they're equally, if not more so, important in the development of film as an artistic medium.
Anyway, long story short, I vote for Dreyer's The Passion on Joan of Arc from 1928. Anyone else have favorites?