It's the one film that most of my students who take the class haven't seen, and after they see it they are blown away by how good it still is 70 years later.
But I have to go with 1954's "Rear Window." Jimmy Stewart and Grace Kelly are the amazing stars for this thriller about a man confined in a wheelchair who sees a murder from his apartment. The whole film takes place in the apartment, the camera never leaves the room. It's the one film that most of my students who take the class haven't seen, and after they see it they are blown away by how good it still is 70 years later.