I was more disappointed by what the movies added than what they left out, since I understand the time restraints but I literally can’t watch Faramir trying to take away the ring, Gollum tricking Frodo, or elves in Helm’s Deep. Because they just didn’t happen, and they’re not good.
Although IDK if it’s fair to say that since Tolkien wrote the trilogy as one book but the printing companies usually print them separately because they are huge.
My favorite standalone book is The Hobbit and my favorite series is the How to Train Your Dragon series and my favorite trilogy is The Lord of the Rings. Don’t make me choose a favorite book!