I have a feeling that this list might be dominated by Lang...
20. The Testament of Dr. Mabuse (1933) d. Fritz Lang
This film feels like a flipside to M. Essentially it's about a raving lunatic who plots out schemes in his insane asylum cell. Unfortunately these plans keep coming to fruition despite Dr Mabuse's incarceration. This is a fascinating blend of social commentary and proto-action movie, with our main protagonist having to join forces with local gangsters to get to the bottom of an ever expanding conspiracy. With shoot outs, evil hypnotists, and a finale at an exploding power-plant this feels like a proto-action film, but it's also laced with a general distrust for authority that earned it a ban in Germany by the Nazi's and finally forced Lang to flee from Germany after he was offered the ultimatum of becoming a Nazi filmmaker or be denied distribution. It's also perhaps one of the most handsomely shot films of the 1930s.
This actually leads me to an interesting point. The 1930s are a difficult time for world cinema. The major cinematic players of the time were the US, UK, France, Germany, Italy, and Japan. Despite certain directors who outright refused to toe the party line (like Lang in Germany and Ozu in Japan) the cinema of Germany, Italy and Japan is hard to qualitively represent on this list because as great a director as someone like Frederick Pabst or Riefensthal are, it's difficult to put the work of a Nazi or Fascist filmmaker onto this kind of list.