Very difficult to pick just one, and it would depend upon the criteria to be applied to "favorite."
For example, there are movies that I would include in my list of favorites which I really wouldn't care to see again. These are movies that have had a profound impression on me which could not be duplicated through a second viewing.
Then there are the films on my list that I can watch every time they are available.
This sub-group is pretty extensive, but the film that jumps first to mind is Tombstone.