As far as classic horror movies go The Cabinet of Dr. Caligari (Das Kabinett des Doktor Caligari) is the oldest -possibly most important too- of the lot. It was, after all, shot back in 1920 and could easily be the first horror movie ever created, though I really can't be sure of such things. Mind you, most the internet does believe this to be the case, so I guess further research isn't what matters right now.
What actually matters, you see, is that you dear lovers of horror, German expressionism and wild, romantic film-making absolutely watch this masterpiece. The film is now happily a public domain work of art and can be freely and very legally downloaded from the Internet Archive.