Oh wow Needful things, what a good call. I actually quite enjoyed that (minus the ending) but the adaption was just terrible which is sad. Stephen King is just so hit or miss. IT was amazing (minus the ending) and they did a great job (minus the ending). But of course it is allll forgiven when you take The Shawshank Redemption and Stand by Me (The Body) into account.
If we're naming worst ones, then I feel compelled to nominate the last Anne of Green Gables movie, I think it was called The Continuing Story of Anne. They butchered it so thoroughly that the only thing that remained the same from the books to the movie were the names of the characters - every other detail had been changed. I think the company that produced it must have had some old script lying around that they couldn't figure out what to do with, so they just slapped the names Anne and Gilbert on the main characters and hoped we wouldn't notice that it was nothing like the books.
Lord of the Ring's is right up there, of course, but I also like the adaptations of Jane Austen classics, like "Emma" (w/ Gwenneth Paltrow), "Clueless" (w/ Alicia Silverstone), "Pride and Prejudice" (w/ Kiera Knightly), and "Sense and Sensibility" (w/ Emma Thompson and Kate Winslet).
I liked the adaptation of "Harry Potter and the Prisoner of Azkaban" and the TV adaptation of "Dead Until Dark" w/c is "True Blood" in HBO.
I rather liked the Telemovie adaptation of "Ballet Shoes" on the BBC, too.
Lord of the Rings here... the books and movies are now forever linked within my mind (I even have an upcoming post reviewing my reread of The Fellowship of the Ring which gets into that line of thinking some more).