Is there a movie you think really nails the book, or maybe even improves on it? Is there anything you were shocked to learn was based on a book?
My roommate recently read Jaws and he said he can’t believe it was made into a movie, and I attempted to read the first book in the Dexter series and felt the same way.
The obvious answer is The Lord of the Rings. The books are better, but the movies were probably the best you could hope for. I did like Blade Runner more than Do Androids Dream of Electric Sheep, though it was a very loose adaptation.