It's rare for me to prefer a movie to the book, but there are a few:
Marathon Man is a fine book, but an outstanding movie I think. Probably Dustin Hoffmann's best performance, Laurence Olivier in fine form, with Roy Scheider and Wiliam Devane also strong support. Very atmospheric and genuinely terrifying.
Silence of the Lambs I thought was nothing special as a book, but became a great movie. How rare is it for Jodie Foster to be outperformed?
Lord of the Rings isn't a fair choice probably, since fantasy is not a favorite genre of mine--I don't think I even finished the book. But I'll watch the movie a number of times in the future no doubt.
Sling Blade--OK, I haven't read or seen the play, but it would have to be truly outstanding to be more captivating than the movie, a
tour de force for Billy Bob Thornton.
Don't Look Now may be the most enigmatic movie I've seen, I certainly found it much more captivating than DuMaurier's book.
The Hunt for Red October is a fine thriller movie I think. I don't enjoy Tom Clancy's books, this is one case where stripping out much of a book's detail is an advantage I feel.
Total Recall isn't fair either, since it's based on a good short story. One of the most interesting sci-fi films, and a surprisingly good performance from Arnie to boot.
Where Eagles Dare and
The Guns of Navarone are I think the two Alistair McLean movies which surpassed his usually strong books. The Alpine scenery is stunning in the former, no way describing it can compete with showing it well.
I'm sure there are more, especially where a mediocre book became a decent movie... but my brain is playing misty for me
Looking at the list above, the common thread seems to be strong acting and/or strong directing on the foundation of a strong screenplay.
But in general, I imagine it's very difficult for the combination of hundreds of variable movie talents to outshine one talented author?