Movies that were better than the original book (all Sci-Fi / Fantasy. I don't know why):
Bladerunner (Do Androids Dream of Electric Sheep?)
Is the movie better? I'm going to say tentatively that Yes, it is. Yes, the book is remarkable, but the movie? It remains the pinnacle of the Sci-Fi movie genre ∴ it is better. Sue me
The Prestige : by Christopher Priest.
The book is very good, but the movie is better formed. It ties up all loose-ends elegantly while still leaving some sneaky paradoxes.
Christopher Nolan did the rewrite. I read he is going to redo / continue Bladerunner
Nolan's work on Batman is awesome. He uses different elements from different strips to create new stories. Much like my next choice...
The Thief of Baghdad / The Arabian Nights:
I can never get into the book
Logan's Run: A clear example of the script-writer taking what is good from a book and chucking the rest.
The 13th Warrior:
The movie was great fun and the book left me cold (I couldn't get into it).
This is actually the reverse. The book adaption of the film is by Orson Scott Card and he does some really good characterization. A really good writer ( or used to be. Sorry Orson!).