Citizen Kane, of course. "That's the official answer," according to Roger Ebert, in a very thoughtful article on what is a great movie and what is a critic's favorite movie.
Our Ben is no prisoner to fashion, though, as evidenced by the poster for I Am Legend, shown below. That is Ben's blurb at the top:
I cannot help but wonder at what point some ad exec will find this to be just a bit too hyperbolic and unbelievable. There comes a point when people see through this sort of nonsense and won't believe anything on the poster. Unfortunately, fawning and self-congratulations will get you very far in Hollywood.