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Just to be fair, it should be pointed out you couldn't really use the same structure in the movies as in the books: the first half of TTT book following Gandalf, Aragorn and company and the second half following Frodo and Sam. It just wouldn't work cinematically. Cutting back and forth between the stories was the only way to do it in the films. Given that you are forced to move the whole Cirith Ungol episode into the third film because chronologically it takes place at the same time as the Battle of Pelennor Fields (look at the Tale of Years in the Appendices of ROTK).
That said, I agree TTT is the weakest of the three films and I think they should have tried harder to stay closer to the book.
The Return of the King was quite a disappointment for some reason. It had many wonderful moments, but it also featured just as many that simply didn't feel right. Make of that what you will, my vote goes to TROTK.