Forrest Gump. The movie is legitimately great, but isn't remotely like the book. Part of what makes a good adaptation is how close they stay to the original source material. The novel Forrest Gump is basically a pure wacky comedy with little to no drama or pathos or the heart of the movie. Stick to the source material or GTFO! :P

The biggest offender is Jurassic Park. Hands down, the worst adaptations of ALL TIME. Massive changes from the book to screen. Biggest change was in the book, the lawyer character was one of the good guys, while in the movie he was the typical sleazy lawyer. I guess, in a movie with DINOSAURS, having a lawyer that's actually a hero was too fantastical for Spielberg. This was officially when I started to turn away from Spielberg.

Other bad adaptations: Any Tom Clancy novel, minus Hunt for Red October. Most Steven King books, with the exception of Shawshank Redemption and Misery.

Now some good adaptations. First Tarantino's Jackie Brown. Despite changing the main characters race from white to black, and adding his own spin on the dialogue, it's a very mature adaptation for such a young director. Especially when you compare it to Steven Soderbergs adaptation of another novel from the same author released in the same year with one of the same actors playing the same role in a unique cross studio crossover. You almost never see the same actor from one studio's film play the same character in another studio's film, even a cameo, but since it was in the book, Tarantino said go for it, and did it free.

And the best adaptation to date...Sin City. Rodriquez's main weakness has been his writing. When you do as many jobs as he does, something has to suffer, and in his case, it's his writing. So when you start with something that's already well written and you decide to film it AS WRITTEN...that's as good as it gets. The dialogue in the movie is nearly word for word from the comic. The comic panels are storyboards for the film with many looking almost exactly the same. He even shot all the comics in full and then released the full edit later. The more faithful you are to the source material, the better a movie you will make, and this is as faithful as it could possibly get.