I would say Star Wars and Empire are. I never like answering that question because I like a lot of movies, but as a default I say the original two Star Wars films. It's probably true, as I watch them every few weeks/months for my entire life and obsess over them to the point of writing a textbook on their origins, so it would be hard to argue otherwise. I don't know of any film I've watched even half as much as ANH. I'm sure I have literally seen it hundreds of times by now, and I'm still not bored.