Land and Freedom, in its entirety. It's a film about a young English Communist who goes to fight the fascists amidst the Spanish Revolution as a member of the POUM militia. He sees both the reality of a people's revolution and the consequences of Stalinism. It's directed by Ken Loach, who also directed Bread and Roses and The Wind that Shakes the Barley. Subtitles will help a lot if you don't speak Spanish.
Everything you ever wanted to read about left-wing political theory but were afraid to look up. [more inside]