Man With a Movie Camera (1929) aka Chelovek s kinoapparatom dir. Dziga Vertov
Vertov's experiment is excellent but it lacks the immediacy that I prefer in a film. There is hardly a plot, but a Joycean stream of events nonetheless that could prove interesting on repeated watching. The commentator on the DVD mentions Walt Whitman repeatedly and if you are a fan of his--I am not--you will enjoy this paean to the 20th century cityscape.

The Cabinet of Dr. Caligari aka das Kabinett des Doktor Caligari (1920) dir. Robert Wiene
An intriguing work overall, this silent film will appeal to fan's of Nabokov's Pale Fire (and vice versa). Both contain an it-was-all-a-dream ending that fails to convince you completely. So you are left to sort through the gamut of interpretations. This is a horror film, but it's not the murder but the madness that is horrifying. Interesting note in the commentary: German expressionism was intended to provide an escape from the trials of life in the Weimar republic!