Them! (1954)

Giant ants, played completely straight, and that is why it works. Gordon Douglas directs like a police procedural, so the strange discoveries in the New Mexico desert and the slow piecing-together of clues build real tension before the first ant appears. It was the first of the decade's big-bug movies, with radiation from the first atomic test blamed for the mutation, and its effects earned an Oscar nomination. The final hunt through the Los Angeles drains is still a terrific set piece.