To Kill A Mockingbird. Brilliant perfomances from Gregory Peck and Mary Badham as Atticus and "Scout". Scout's narration as a grown up and the sleepy nostalgic atmosphere are brilliant too.
For me it almost always seems to be the one I encounter first. A few films that I consider superior to the book that they are based on include: The Exorcist, 2001, Jaws and The Shinning.
Lord of the Rings is a difficult one for me. Loved the books when I read them years ago, but also loved the films. If nothing else, the films captured how I visualised Middle Earth in my minds eye.
Carl Sagan's novel, "Contact," has some good ideas, but still kind of mediocre as far as novels go. After all, Sagan was a scientist. Robert Zemeckis' adaptation is far superior; probably one of the best sci-fi movies ever made.
My husband's youngest nephew, Leif Enger, had a best-seller a few years back that is being made into a movie. Brad Pitt is producing it and Billy Bob Thornton is one of the characters. Interesting bit of trivia there! He has just come out with another book as has his older brother Lin. But don't know if they will make it to the top like "Peace Like a River" did.