I'll start this off with a couple of my own:
Best - Although it's kind of cheating (since it was technically a mini-series), Band of Brothers ranks up there as one of the best movies from a book. Granted, it took ten hours worth of footage to do it, but they did an incredible job of putting that on screen and not leaving anything out.
Honorable mention - Fight Club by Chuck Palahniuk. In all honesty, it's probably the only movie I've ever seen that came from a book where you actually need to see the movie to understand some of what is going on, as well as vice versa.
Worst - I made it all the way through the abomination known as Syrianna before I found out that it was supposed to be a film adaptation of the book See No Evil by Robert Baer. I read that book not more than six months before seeing the movie, and had absolutely no idea they were connected until the credits rolled. The book was an excellent series of adventures experienced by the author in his twenty-plus year CIA career. The movie was... George Clooney wanking.
Honorable mention - Anything by Michael Crichton. I really enjoy his books, but his movies just plain suck (with the possible exception of Jurassic Park). They always completely miss the cool factor of his books, which is usually some kind of revolutionary technological gizmo or invention. While the movies end up being Demi Moore in a bra trying to rape Michael Douglas. Riiiight.
Your thoughts are not only needed, but necessary if you are to avoid being killed by a Ninja.