If the question were about framing or depth, my first inclination would be The Searchers (1956). If someone were to ask what I would recommend for teaching a class on editing and auteur theory, I would suggest Citizen Kane (1941). My lists, filed by theory and genre, include films that run the gamut, from the elite to the avant garde. Over the years, my background in studying film has cultivated my memory into a well-curated database of movies.