There have been so many…
Typically I never like the movie as much as I do the original book, but I must admit that the Harry Potter movies have done a fairly good job in capturing most of the essence of the books. The books are much better, however.
The movie version of Eragon totally missed what made the book so good, the process of boy and dragon both maturing and the relationships between the villagers. Instead it focused on fights and special effects.
LOTR was pretty good but as with any movie much had to be left out due to time constraints and some characters were definitely Hollywoodized.
Starship Troopers the book is totally different from the movie and much, much better.
The original Roald Dahl book on which the two movies (Willie Wonka and the Chocolate Factory with Gene Wilder and Charlie and the Chocolate Factory with Johnny Depp) are based is in many ways actually simpler than either film. While I like both movies, I did enjoy the expansion of Willie Wonka’s back story in the Johnny Depp version. It also takes some of its details from the sequel, Charlie and the Great Glass Elevator. More importantly, the Johnny Depp film captured a bit more of the original creepiness of the original book. The Gene Wilder film, while a lovely hallucinogenic ride, upset Roald Dahl so much he could never watch it all the way through.
Interestingly enough, the Gene Wilder film was originally a box office flop, barely earning back what it cost to make. The Tim Burton/Johnny Depp version in contrast was your basic blockbuster.
One thing I found interesting was that in reading Dahl’s autobiography “Boy” I found out where he got the idea for it all. His school experiences weren’t particularly pleasant and one nasty memory he had was of a candy store proprietor who was weird and a bit creepy. Dahl and his pals managed somehow to put a dead mouse in among the candies at one point so I suppose that the proprietor had a right to dislike him, but he ended up being beaten rather badly by the school headmaster at the request of the store owner. Many of the nastier teachers he had ended up later on as villains in his books.
One book I found benefited from being turned into a movie was Stephen King’s novel The Shining. The book had more detail (and in some cases too much detail) but the movie crystallized the story. Of course, Jack Nicholson’s performance was classic, but after seeing the movie some aspects of the book became clearer and I could go back to read it later with much more enjoyment.
If you go to http://www.mcpl.lib.mo.us/readers/movies/book.cfm you will find a lovely list of books turned into movies since 1980, some 1250 films. You can go down the listing and see if any other film/book combos stand out.