In no particular order: Psycho: I've lost track of how many times I've seen this film. The noir vibe, the strong performances, great soundtrack and Hitchcock's manipulation of the audience make me keep coming back to this film. Butch Cassidy and the Sundance Kid: The chemistry between Newman and Redford is amazing. I love the soundtrack and it's a beautifully shot and edited film. Miller's Crossing: Gabriel Byrne's Tom Reagan is my favorite anti-hero. Also, it has a great soundtrack, cinematography and performances (esp. Turturro and Harden). In the Mood for Love: I feel that every frame of this film is a feast for the eyes. Reservoir Dogs: This was the first movie I saw in the theater that I immediately wanted to watch again.
May be unpopular for this but I've seen neither Psycho nor Resevoir Dogs. I love both Hitchcock and Tarantino, so I have no excuse as to why I never watched them apart from "never got round to it". Every time someone asks if I've seen these films and then look astounded I haven't a little part of me dies inside, yet I keep forgetting to bloody watch them!