I saw these in a history of doc class, all were excellent, short rarities to check out:



Las Hurdes or "Land Without Bread" is a Bunuel doc he made after Un Chien Andalou, which sardonically makes fun of poor people living in a forgotten Spanish village.



The Blood of the Beasts: a film made in post-war France about slaughterhouses in Paris. Very graphic, but in a surrealist tradition.



Night and Fog: probably the most popular and well-known film made about the Holocaust, incredibly graphic but well-told like a memory. Pretty bleak, but great stuff.