Don't get me started... Ah, too late!
The Fellowship of the Ring is an excellent film, pretty consistently following the story while removing confusing elements like Tom Bombadil (some scenes, like the Moria stone troll fight, could have been a bit shorter, but overall it was really amazing).
The Two Towers started showing the early signs os Jackson's "interventionist" approach (which later culminated in the Hobbit films) with adding numerous elements like Aragorn's side trek and Gimli's and Legolas' banter -- but again, it overall added to the story without removing too much of it.
However, with The Return of the King he completely blew it, changing some important elements like Ents' motivations and, above all, the role of the Dead Men of Dunharrow. In the books, they're a formidable but reluctant force which has a strong but limited impact on the story -- they aid Aragorn's attack on the ships, scaring most of the Umbar's forces into drowning and allowing him to increase his army with the captives on the ships, but then he releases them and they never reach Pelennor.
In the film, they are turned to a deus ex machina which sweeps the battlefield and the streets of Minas Tirith, killing all the enemies and not touching the "good guys" (the book shows that the Dead Men scare everyone equally; the "good guys" survive because they are captive and chained to the ships, which probably needed a lot of cleaning afterwards). That scene reminded me of those commercials where a green goo destroys all the toilet bacteria in one powerful sweep -- in my opinion, that change totally destroyed the premise of the book and turned it from "The Return of the King" to "The Return of some Guy with Toilet Cleaner".
/rant