M (1931)

Directed by Fritz Lang. Starring Peter Lorre.

I kind of regret only having one Lang film on this list, given that he's one of the pioneers of cinema as an artistically-viable medium, but I guess if I had to choose only one, this would be it. Lorre plays Hans Beckert, a seemingly harmless man who is also a child-killer. As Hans' reign of terror grips the city, the police try to solve the mystery of the killer's identity while the criminal underground decides to hunt down the man whose crimes have led to an increase in police on the streets. In addition to seemingly inventing both the cinematic incarnations of police procedural and the serial-killer mystery, Lang was also ahead of his time in his use of sound- compared to most films of the time which were wall-to-wall dialogue, Lang uses dialogue sparingly, relying instead on silence, sound effects, and Lorre's shrill "Peer Gynt" whistling. In the midst of everything else, Lorre is magnificent as a man who feels an overwhelming compulsion to commit his heinous crimes, helpless to control himself ("I must!" he confesses to an inquisition), and he somehow becomes a pathetic figure as everyone else in the film is bearing down on him.