Winsor McCay was an early film animator and "America's greatest cartoonist". I especially like his clouds and his waves. The movements of his figures seem more natural than some of the later animations I've seen.
Little Nemo (1911) is a partially hand-colored short:
See his comic strip characters come to life.
Gertie the Dinosaur (1914):
The Sinking of the Lusitania (1918):
This was one of many WW1 propaganda efforts but the first animated one I recall seeing.
The Centaurs (1921):
Flip's Circus (1921):
The Flying House (1921):
More from the "Rarebit Fiend" series can be found here.
I got a comment below from someone who has a great homage site to McCay: Meeting McCay. The post on 12/4 is on Maurice Sendak, noting the obvious McCay influence on his work. We were always big Sendak fans when the kids were little but have just now discovered McCay. Earlier in the month he notes McCay's ties with Buster Keaton. It is a small world.