While the Princess Bride is a book that's a good book, the movie is much better as a movie. I think the Peter Falk framing device works a lot better than the "good bits" metafiction framework, and the casting agent for the film should have gotten a damn medal.
I completely agree with this - especially the medal part. There isn't a character in this film that wasn't perfectly cast. This is one of very few films that I will watch over and over and never tire of. The only thing I would change is I'd improve the special effects for the RUS's, but the fact that it's so obviously a man in a giant rat costume both lends to the movie's charm and detracts from it.
My other choice would be Jurassic Park. I am sooo glad I saw the movie before I read the book, because without knowing what I was supposed to see, the movie was amazing and is still one of my very favorite movies. The book was excellent, but if I'd read the book first, I'd have been really disappointed in the character and plot changes.
And on Lord of the Rings, I love the books. The Hobbit, too, which I've actually read a few more times than LotR. However, this is another case where a combination of the director, screenplay author and the film editior know exactly what to cut to keep the movie true to the book without making the movie boring. Both Jurassic Park and LotR would have been really dull movies if they were filmed exactly like the books. Sometimes it takes creative license to marry the books and the films. In Jurassic Park, Michael Crichton wrote both the book and the screenplay, so he clearly understood that what makes a great book does not necessarily make a great movie.