"Return of The King is the most enjoyable because in the structure of the movies, it is nothing other than pay-off, there is no more setting up to do, no more exposition, no more introducing characters.
The pay-off is very character-based. It is action-orientated as well, but all of our characters have been pushed to a point where their life and death depends on what happens in the third movie.
It is very emotional, and from an actors' point of view it is very enjoyable to work on, because they were able to play some pretty intense drama.
From my point of view it was always great, because we were heading toward an ending, a climax which we never had in the other two."

This "pay-off" — as Jackson put it in the above quote lifted from the IMDb — is singularly rewarding because 'The Return of the King' is the film that pays tribute to the extremely hard work the leading members of the cast put into the difficult task of character development throughout their years spent on the trilogy as a whole.
The film features some of the most spectacular sets and finest camerawork, using New Zealand's magnificent mountain scenery, in the whole of Jackson's 'Lord of the Rings'. For anybody seeking one of the best bunch of picture (computer "desktop wallpaper") places, there's a treasure trove at Minas Tirith ('Ship of Dreams' site*), while the IMDb links to many others.
From a host of reviews of 'Return of the King' and some interesting comments all over the Web, it struck me that almost everybody who didn't enjoy or admire the film took issue either with Jackson's view of Tolkien's tomes or with weaknesses and thin areas in the work of the South African-born professor of Anglo-Saxon (Tolkien.co.uk) itself.
But I haven't revisited 'The Hobbit' or 'The Lord of the Rings' for such a long time that I can no longer say whether the trilogy is strictly faithful to the books. When I did read them as a child, I found them hard, sometimes turgid, with long and very tedious bits which Jackson certainly left out of his films, and yet still a great piece of literature.
Some people, like "systematic theology" scholar J. Ligon Duncan III, find the whole thing "shot through with redemptive metaphors, Christian virtues, veiled references to divine providence and Christ-analogies" (Christianity.com).
Tolkien loathed allegory, as Duncan himself points out, and I wouldn't push the Christian case very far myself. But Bradley Birzer, in his 'J.R.R. Tolkien's Sanctifying Myth' (Townhall review by Tim O'Bryhim) even saw it all as a "theological thriller"...

When Jackson's work first hit the screens, I saw "explanatory" bullshit drawing direct parallels with contemporary history and particularly the fundamentalist Bush administration's crusade against the so-called "axis of evil". Take this, for example:

"...there's a profound parallel between this fantasy tale and what's happening in America today. We all know this one: A brutal terrorist attack shocks a kind but self-centered people into the wide-eyed realization that their nation is the target of intense hatred that's crept up on it for years. (...)

"Lecture topics include (...) how unseen astral forces amplify and manipulate global conflicts in an all-consuming battle of darkness versus light" ('Destiny of America') — always so reassuring to know!