I don't think that there is a best one. Very famous ones are Triumph of the Will (which had pretty influential camera and film technique), Der ewige Jude or Jud Süß (one of the premier anti-semitic films) and the propaganda movie about the Ghetto Theresienstadt/ Terezin (which wanted to show how good Jews have it in their ghettos).

Start up Pirate Bay and download 'em. A lot of stuff is also on youtube. They don't have any copyright anymore as far as I know.