I agree with too many other posters to mention!
LOTR - I dreaded the films coming out as I love the books but thought they were brilliant adaptations. I loved the way they got some of the best of the narrative in by translating it to dialogue - like Gandalf's speech about the swift sunrise. Lots of film adaptations fall flat because some of the best of the writing is lost but LOTR got round that.
The Last Unicorn - loved both book and film equally
The Princess Bride - the film narrowly wins
The Dark is Rising - saw 5 minutes and nearly threw something at the screen!
Children of Green Knowe - loved both
Box of Delights - ditto
Time Traveller's Wife - loved the book, found the film tedious
Sleepers - good book and film
Alex Cross films - agree that Morgan Freeman just isn't right
My Sister's Keeper - good book, pointlessly altered film
Twilight - books were enjoyable rubbish, films were just rubbish
Harry Potter - books much better than films, particularly the last one
The Lovely Bones - daren't watch it!