I think it depends on what you mean by a great film adaptation. If you are looking for a really good movie or if you are looking for a really good movie that is really true to (at least the spirit of) a really good book.

I don't know if you need to put the 'really good' in really good book.

The Princess Bride is one of the closest adaptations I can think of. While the book is good, it doesn't rise to the greatness that the movie does. Why is it such a good adaptation? Well the author of the book did the screenplay, and the author has some chops at writting them. William Goldsmith has an academy award for Butch and Sundance.

To tell the truth, the book reads like a novelization of the movie, even though it was published 10 years before the movie was made.

Ford
Was on vacation