Often considered the first true horror movie – "The Cabinet of Dr. Caligari" has a sinister edge that can still draw blood today. The plot has an evil doctor commanding a young man he has under his hypnotic control to do his bidding – which often includes, as you might have guessed, murder. In addition to the nightmarish thriller plot, other Expressionistic elements at play here include wild and out-of-whack set design that emphasizes that something (or maybe everything) isn't quite right, and shadows that were literally painted onto the set when real shadows weren't dark enough.
The other granddaddy of horror (along with "Caligari") is "Nosferatu," FW Murnau's classic version of the story of "Dracula." The sets and visuals aren't as unhinged as in "Caligari," but the power to shock is almost as potent, and Max Schreck gives one of the great silent screen performances as Count Orlok, the vampire.
"The Last Laugh"
Not all German Expressionist movies have to do with vampires and murder. This one is about the simple horrors of just getting by. Famous German actor Emil Jannings plays a doorman at a fancy hotel who puts everything he's got into his job. So when his job gets taken away, he loses practically everything. Director Murnau (again) shows this in purely visual terms, with (almost) no title cards. His "unchained camera" glides along in graceful long takes that make the movie seem much more modern than it is.
Now we have vampires again. Directed by Carl Dreyer, this is one of the few German Expressionist movies that is technically a sound film, although much of the dialog and plot is relayed through title cards anyway. The movie is full of memorable (and terrifying) sequences, including a nightmare in which a character sees himself in a coffin, and a man being suffocated to death by pounds and pounds of flour.