Citizen Kane

It's an exceptionally good film. "Best movie ever" is a lot of nonsense of course, one can hold out for The Godfather or Seven Samurai or whatever and not be wrong.

It just has so many great little touches though. I love the forced perspective shot where Welles walks towards the windows and you're surprised by how far away they are. Or the line reading on "Everything you hate."

"Best movie" probably also refers in part to the filmography, which was exceptional for the time. His inexperience actually helped him there, because he did things with shots and cameras that were commonly known as "impossible to do" at the time.

He also had Gregg Toland as his cinematographer, Toland being one of the best cinematographers who ever lived. Toland had actually done some of the deep focus shots so strongly associated with this movie in earlier films, like The Long Voyage Home.