If I could only watch one movie to open my eyes to the art of film, what would it be?

Like pretend I’m dying or something lol, or that I love art but have never watched a movie before, or that I think that the best movie ever made in the history of movies is Iron Man.

Which movie would I have to watch to appreciate the art of film? Tough question, but remember I can only see one. If it’s too difficult, you can name one from different genres.