What makes a movie great and can a film be better than the book?

It's something that I often think about all the time. What makes a film great, is it necessarily the dialogue alone or the characters or the idea? I have a thought, it might be the dialogue and how many set-up points there is in the script or is there no real answer? Everyone out there let me know your ideas.....People tell me all the time a book before the film is much better than any film remake but is this always true?