I actually prefer The Hobbit to The Lord of the Rings (and in fact I wasn’t aware the latter even existed for many years after reading The Hobbit). At first I thought that maybe this was influencing my hatred of the film version, since the story was a little closer to my heart than LOTR, but I’m fairly certain it’s the movie at fault, not me.
Even though I didn’t get to LOTR until later, I still read it and was a fan of it long before those movies came out, and their divergences from the book didn’t bother me because I felt they still captured the general spirit of the books- the emotional impact, the essence of most of the characters, the epic scope of the journey, etc. There were a lot of things one could find fault with, but I don’t feel it was a perversion of the source material the way Christopher Tolkien seems to have.
The Hobbit, on the other hand, seemed to get everything wrong. Serious moments were made cartoonish, pointless padding was added in even while important scenes were truncated, characters didn’t feel right, quiet moments were made loud and bombastic, emotional moments were made shallow and hollow. I understand they had to make changes to convert it into a film, just like LOTR, but this time those changes destroyed the spirit of the story rather than just altering the details of how that story played out.
As for an instance where the book wasn’t as good as the film, The Shining comes to mind. I didn’t particularly like either the movie or the book, but the movie at least held my interest, has some iconic moments, and is well directed. The book was an incoherent bore, and that’s coming from someone who usually likes Stephen King.