I haven't read many books vs movies.. I usually do one or the other.
Game of Thrones -- A TV series in truth, not a movie.. and although the books are technically better, the series cuts out a lot of the 'boring' parts of the books for those that don't like world-building. They stick to the books rather well for a series, and it brings the characters to life and puts some faces to them, which I particularly liked for Eddard and Joffery. The action, the predatory relationships, and a lot of the grit and gore seems to be in the series so people without the time to enjoy the books can still be in on the craze. I think that makes it at least on-par with the book via reaching out to the masses.
I have this same feeling about LotR. While the books are better, the fact that there are faces to the characters, and enough beauty to show the world the author was seeing in his head and cutting out a lot of world-building traits allowed people to fall in love with the world without muscling through the tomes.. and people can always go back and read the books after they fall in love.
I am Legend was not better than the book ONLY for the switched up ending. If they had kept the ending true to the book it would have been an amazing movie. People wanted *us* to be right. The whole point of the book was to (spoiler!) show that humanity was the monster. He was going around killing and murdering. He thought he was being the good guy, and he realizes he was a monster being hunted down. It would have been a shocking, profound ending...
One Flew Over the Cuckoo's Nest -- Better than the book, completely agreed. The book was fine as it was, I enjoyed reading the thoughts of the Indian.. something you miss out on a bit in the book. But the lobotomy was SO POWERFUL. Seeing him in a vegetative state was gut-wrenching, and the visual had an effect the book never could.
I'm assuming Jaws is the same way.. the book could never portray the fear that the movie created.
Silence of the Lambs I never read. But the movie created a sense of fear and creep and mystery that I feel the book probably could not have accomplished in the same manner. That movie was very powerful.