Eras and the development of camera techniques/special effects could be another way to go. We could start off with some of D.W. Griffith's films, like
The Birth of a Nation from 1915. It brought film as an art form to the masses, and advanced the way films looked tremendously.
Another film that I love is 1905's Le Voyage Dans La Lune, which is only 15 minutes long, and public domain! It's sweet, creative and full of heart! It's one of the first films that had an artistic "statement". Before it, films were mostly cheap and for the "working masses". Radio was seen as the smart media.
If you want to see Voyage, here is an amazing newly restored edition with an original soundtrack by the French band Air! You want to see it ;-)
[video=vimeo;39275260]https://vimeo.com/39275260[/video]
New Hollywood could be cool. The new filmmakers after the first mavericks of American cinema. Often very political and opinionated, there's a lot of great stuff there!