My favourite adaptation has to be Peter Jackson's take on J.R.R. Tolkein's The Lord of the Rings - the vision conjured up seemed to match what I had thought when I first read the trilogy.
It was wonderful "seeing" The Shire. The darkness of Mordor was vividly brought to life too.
I've also loved the adaptations of Terry Pratchett's Going Postal, Hogfather, and The Colour of Magic.
I would love to see an adaptation of Men at Arms and Raising Steam.
I still don't understand how you can get three films out of The Hobbit though!
Having said all of that, I am all for film adaptations of books as long as they stay faithful to the book. I don't "get" changing endings, character roles etc. It makes it a different story to the one the author originally intended and I really can't see the point of that.