Lord of the Rings by JRR Tolkien.
I can easily say this is my favourite book turned movie, because while I wish I had never wasted my time reading this boring testosterone-overloaded epic, I really really loved the films.
I agree with every single change Peter Jackson and his writing team made to the adaptation to make it into a better movie. Every single thing: all the timeline changes (Boromir dying at the end of the first film, Shelob appearing in the final film etc); making Frodo younger; eliminating Tom Bombadill; making Arwen more important and having a bigger presence; making Sam and Frodo’s relationship more interesting with more drama; and many, many others details I can’t be bothered writing here. There are a lot.
Even though I am a fantasy reader and a fantasy writer, I found the book boring and uninspiring. The films, on the other hand, were excellent, the best non-faithful film adaptations ever made. I own the Extended Editions, as I find them even better than the theatrical releases. I don’t see how another film adaptation could ever be made, and be made better than these.
And finally, I need to express the fact that yes, Tolkien is a genius and yes, he is the father of modern fantasy, and yes, I do understand that he invented orcs and was the first person to make elves tall and willowy (as opposed to Santa’s elves) and yes, the story is amazing and the characters are awesome. The films couldn’t have been so wonderful if it weren’t for this source material. However I, personally, did not enjoy reading the books. It’s just not my cup of tea. It was very dry and methodical – I get that from my academic readings, not my fiction. So don’t get all up on your high horse and tell me I don’t ‘understand’ fantasy or that I’m not a fan. The films were simply better.