That is an impossible question to answer. I love so many different movies, and so many different types of movies for so many reasons. It would be impossible to narrow down just one movie and call it my number one favorite. You have to judge a movie not only based on the screenplay and the acting, but moreover by how a movie makes you feel when you see it for the first time. Great movies are about emotions they ressurect in you.
Great movies have great music.
Great movies have music that is so fitting that even if the song is not one of your favorites when you hear it on the radio your mind immediately jumps to that scene in the film. This is why I can't narrow down to one favorite film.