M

I watched this movie on Halloween and it scared the hell out of me. M is noted for being the very first serial killer film, the first film to ever use a musical theme to signify a character and was Lang's first sound film.
Made in 1931, this German drama-thriller directed by Fritz Lang, was written by Lang and his wife Thea von Harbou. M is supposedly based on the real-life case of serial killer Peter Kürten, the "Vampire of Düsseldorf", whose crimes took place in the 1920s, although Lang denied that he drew from this case. When the police in a German city are unable to catch a child-murderer, other criminals join in the manhunt. The murderer, played by Peter Lorre, lures his innocent victims to their death by whistling the tune "In the Hall of the Mountain King". Lorre himself could not whistle – it is actually Lang who is heard. This association of a musical theme with a particular character or situation, a technique borrowed from opera, is now a film staple. Before making this, Peter Lorre had mainly been a comedic actor.
The film has a very sour vision of contemporary life in Germany. This is probably due to the fact that Fritz Lang - a Jew - was alarmed at the rapid rise of Nazism and that even his wife Thea von Harbou had become a party member. Not surprisingly, the film was banned by Nazi Germany in 1934.
For me, the scariest part of the movie was near the end when the criminals of the city finally catch the murderer. They threaten to hold their own court for him and you can just see in their eyes- they're about to tear him limb from limb. Lorre did such an amazing job of being terrified, just scared out of his mind. Imagine how the children felt.