I thought that The Loved One was better as a movie but just because the character of Aimee Thanatogenous was more developed in the movie.

For me, the movie Dr. Strangelove edged out the book just because of the scene in the War Room when the camera panned across the table sliding quickly past the book Selected European Targets in Megadeaths. That kind of subtlety can't happen in a book.

The reason that the book usually far outclasses the movie is that it's just not possible to fit a book into the time period that a movie can sustain. Robert Graves's I, Claudius was made into a thirteen hour BBC special. They managed to squeeze in about one half of the first book and maybe a tenth of the second, Claudius the god and his wife, Messalina.