Jurassic Park really is a good contender with its book, seeing as how Crichton tends to have a big impact in the film making process also - God rest his soul.
And yeah, I was going to say LotR! As much as I liked the movies, Tolkien is so wordy that I don't think even the extended editions can really capture all the charm of the books. Still, fantastic movies, and they really did give the books a run for their money.