I like this thread, time for opinions:
As a rule, movies based on video games are almost universally terrible (although I enjoyed Mortal Kombat and a few others). When Halo eventually turns into a movie, I expect it to be in reverse order (Game>Movie>Books).
I agree that Jurassic Park, Fight Club and A Clockwork Orange are definite possibilities for movie-better-than-book, they came to my mind too. I prefer Clockwork without the happy ending, regardless of the author's opinion.
Watchmen as a movie (though certainly not better) was almost perfectly adapted from the graphic novel, and I actually may prefer the movie ending slightly.
Also, The Thing (1982, of course) was a ridiculously good movie, and now I realize that I haven't read the book and need to do that immediately.
After seeing this thread, I also intend to read the Princess Bride and Stardust, and already planned on Hitchhiker's Guide.
In book version I've heard Eragon was mediocre and haven't read Children of Men, but the movies were so bad I can't imagine the books were worse. Chronicles of Riddick (movie) was bad because it was already a sequel to a surprisingly good movie called Pitch Black.
And Lord of the Rings, really? The movies kept the plot well, but weren't even that good, aside from special effects and cinematography. Yes, the books were sometimes slow, but Tolkien's writing is about more than how many pages they spend walking (a bunch), but rather the world he created. The entire genre of modern epic fantasy is still evolving directly from Middle Earth, so much so that new fantasy settings are considered unique in how they differ.