Newsarama has a list of best genre films by decade. They talk a bit about what they mean by "genre" and then name these 4 films as the best of the 1920's:
The Cabinet of Dr. Caligari (1920)
Phantom of the Opera (1925)
I've seen all 4 of these and link above to where I wrote about them.
Newsarama enters fully into the spirit of the thing by naming 11 films and 3 serials from the 1930's as best genre films of that decade. The films:
Dr. Jekyll and Mr. Hyde (1931) (seen, but not since I started blogging)
Island of Lost Souls (1932)
The Invisible Man (1933)
King Kong (1933)
The Bride of Frankenstein (1935)
Flash Gordon (1936; serial and film edit) (seen, but not since I started blogging)
Things to Come (1936)
Snow White and the Seven Dwarves (1938) (seen, but not since I started blogging)
The Wizard of Oz (1939) (seen, but not since I started blogging)
I would kick Bride of Frankenstein off this list, as I'm about the only one who prefers the original, and that would give me an even 10.