The middle parts of movie trilogies are frequently the best ones (Godfather II, The Empire Strikes Back, Spider-Man 2 come to mind). I'm not sure of the exact reasons but it's usually a combination of familiarity with the characters and one of these following two reasons: they try to end with a bang in the third movie and go a little overboard or the whole thing has just run out of fresh ideas. The Two Towers is the best of the LOTR trilogy, but not for any of the reasons mentioned above. The other two movies in this particular trilogy are both excellent movies, both deserving acclaim. It's just that The Two Towers is somehow ... better.
The movie deviates from Tolkien's book the most of three, and it turns out to be a good thing. The scriptwriters move some events around, fiddle with some of the plot lines and the characters and throw in some brand new stuff for good measure. The end result is that while Fellowship gave you the feeling that LOTR is some variety of an action adventure, this movie leaves you in no doubt that it's a true epic.
LOTR is a true epic in every sense. It's a grand story about the passing of an age. A tragedy that consumes so many of it's heroes and leaves the rest scarred. Where every victory comes at a price and sometimes the price is the destruction of something old and/or beautiful. In the end, it is about growing up and losing the magic we believe in when young. Tolkien makes is abundantly clear that we can never go back to how things were and it's this movie that rams the message home.
This movie is the heart of the epic and is successful in transforming our view of it too. The trilogy came to a stupendous conclusion in Return but I'm glad the filmmakers didn't leave everything till the end.
10/10