A few of mine —
A Grin Without a Cat [Chris Marker, 1977] The Battle of Chile [Patricio Guzmán, 1979] Harlan County U.S.A. [Barbara Kopple, 1976] Lucia [Humberto Solás, 1968] Attica [Cinda Firestone, 1974] La Commune (Paris, 1871) [Peter Watkins, 2000]
Not strictly leftist, but Century of the Self is an amazing docu series by Adam Curtis. I watched it when I was pretty lib and I think it did a lot to push me left when I was in university.