Closest: Fear and Loathing in Las Vegas. I just read the book and was amazed that the movie dialogue is literally 100% word for word out of the book, which is cool because the entire movie is so quotable. They did cut out about 30% of the book, but what's in is perfect.
Farthest: Under the Skin. The book is about the ethics of eating other sentient creatures; the movie is about the art of film, with more of the emphasis on using non-professional actors, hidden cameras, and a simplistic, artistic, eerie vibe, and doesn't follow the book at all, other than the general idea of an alien woman killing men she seduces and the ending scene, kinda. I respect the movie for what it is, but I miss the intellectual debate the book emphasied.