Lang, Murnau, Lubitsch, Herzog, Fassbinder. They make up a lot of this list, but hopefully there are lots of other interesting filmmakers for you to get to know. Haven't seen so much recent german film, however, and I have to rectify that!
Just a note on content. Co-productions make decided if a film is German or not an issue, so if you think i should or shouldn't have included something then apologies. As examples, 'Paris Texas', with a german director and part-German finance, but shot in the US in English, is not here; 'Michael' with a Danish director, but shot in Germany is.