Well, I look at it like this:
The Lord of the Rings movies as a whole: I can't look away to the comparison with the book. Peter Jackson just changed a lot of things, and removing quite a few memorable parts from the book. But then again, to make Lord of The Rings as a succesful film, he made it quite good, in not making an overload.
If he'ld literally made the film version of Lord of The Rings: It would be more than three films. :O For that matter, he made an excellent adaptation.
Fellowship of The Ring:
This is quite bizarre to look at. Although it's like the introduction to Middle-Earth, every sequence is cramped up, and very hasty (when you look at the vast distances they travel in Fellowship of the Ring only). But then again, my review is quite critical, since I actually read the books.

So, Fellowship of The Ring is the slowest advancing movie, and therefore the worst of the three.
The Two Towers:
Oho, this is like The Empire Strikes Back in Middle-Earth. The action is taken to a further higher level; more character development, although the most disturbing, and quite unavoidable factor is:
The fellowship is split into three parties, and filming sequences of the three different parties is becoming quite annoying (like the battle of Helmsdeep /WOOP/ change back to Gollem's malfide /WOOP/ back to the sleepyness of the Ents /WOOP/ back to battle. xD
The Return of the King:
Peter Jackson really changed way too much here. But that's not really bothering, since cinematically: everything looks terrific. Every parts blends in so sublime, it's one of the better films of this decade, for sure. The ending was a tiny wittle bit long-going. But very touching.