First of all I think it's impossible to say that any one film is The Greatest. And I think Spielberg is not only very self-important but over-imports his friends, in general, and he was really just giving his old friend a public handjob as these people do. He probably really thinks it's one of his or maybe The General or maybe something else, but it was a nice thing to say about his buddy.
But then, thinking more seriously on the question, it's an awfully good movie. But to say it or any other is The Greatest I would have to watch and/or re-watch thousands of American movies in fairly close proximity; taking into account the context of the time that the films were made and what techniques narratively, artistically, and technically were available and what had been done before them; and compare (and I mean starting over right now so no, not even Spielberg is really qualified) all aspects of the films (writing, directing, cinematography, editing, sound, music, acting, etc.) as well as the sum of their parts... and then maybe I'd have a cloud of a hundred or so films any of which a person could say is The Greatest but people would probably just pick the most celebrated among those and The Godfather would win anyway.
I'm not just trying to be pedantic here, I'm saying There Is No Greatest Film.
And Spielberg should know better. And did but he loves his buddy.
But then, thinking more seriously on the question, it's an awfully good movie. But to say it or any other is The Greatest I would have to watch and/or re-watch thousands of American movies in fairly close proximity; taking into account the context of the time that the films were made and what techniques narratively, artistically, and technically were available and what had been done before them; and compare (and I mean starting over right now so no, not even Spielberg is really qualified) all aspects of the films (writing, directing, cinematography, editing, sound, music, acting, etc.) as well as the sum of their parts... and then maybe I'd have a cloud of a hundred or so films any of which a person could say is The Greatest but people would probably just pick the most celebrated among those and The Godfather would win anyway.
I'm not just trying to be pedantic here, I'm saying There Is No Greatest Film.
And Spielberg should know better. And did but he loves his buddy.