The Cabinet of Dr. Caligari

This silent masterpiece from the 1920s may contain the first twist ending in a film ever. A man, sitting in the park with a friend, recounts the horrific story of the insane Dr. Caligari and his zombie-esque somnambulist (a sleepwalker), who wreak havoc on a town, murder his best friend and kidnap the love his life. As the story becomes wildly more convoluted and incredible, we realize that it is actually our unreliable narrator who is crazy; the institutionalized man is using inmates from around the asylum where he dwells (including his doctor, Caligari) to populate his fantasy. The German Expressionist aesthetic of the film is used to great thematic effect, as well: it features extremely theatrical, stylized set design in the "flashback" sequences as a means to mirror the narrator's unhinged mind and give clues to the unreality of the story. This is the one that started it all.