I personally found Lord of the Rings: The Two Towers to be the worst adaptation of a novel to a film I've ever seen.

Rating: Null/10 (30 December 2002)

SPOILER ALERT
First of all, if you wish to see 'The Two Towers' unspoiled (well, at least that hack job Peter Jackson calls The Two Towers) then I suggest that you do not read any further.
Before I go into what I absolutely despised about this movie (which was the majority of it) I shall first state the few things I DID like about it, because those few things were very well done, and deserve proper credit.
First of all, the portrayal of Gollum. For the most part, Gollum was in character- though he came off as being more of a pity case then the despicable 'Stinker and Slinker' that all readers of Tolkien's works know him to be as. As far as computer generated characters go, he's easily the best I've seen, and WETA should be applauded for that. Andy Serkis also deserves a line, as his portrayal was very good.
The opening fight between Gandalf and the Balrog was carried off very nicely. There's not really much more I can say about it, save that it looked nice, and gave a very misleading opening to the movie.
Two words- Eomer. Eowyn. They were portrayed well.
Finally, the soundtrack- it was as beautiful as ever.
Now, on to what made this movie (no longer an adaptation of a classic, but a way to rope in viewers.) so bad for me that I wanted to cry, walk out, or do both.
Characterisation is head of the list. Now, which butchered character shall I begin with...I believe it was Faramir who offended me the most.

In Tolkien's works, Faramir was a good, noble man. He had honour, and no desire to take the Ring from Frodo. In fact, once he guessed what Frodo was carrying, he told the hobbit to 'Speak of it no more', in case he should feel tempted.
Gimli was a close second in the 'worst portrayal' list. Again, he was an honourable character in the novel- and was reduced to nothing but character relief. What was with his complaining about 'not eating and sleeping for three days and nights' at the start of the film? Gimli would never complain about such a thing! Dwarves in the novels were able to carry heavy loads and run for days. Also, Gimli would not complain when two of his friends were in the hands of the enemy, when he is on his way to help them. Not only was he comic relief- the complaining made him appear shallow and heartless as well.
Theoden was also fairly close to taking the number one spot, but came in at third. He was in the books- guess what, a noble and honourable character! (What IS it with Peter Jackson and Fran Walsh cutting out honour in the second film?) The moment he was out of Grima's influence (note please, that Saruman had NOT magically possessed him like the movie led us to believe.) he realised that there was a problem with his nation, and immediately acted on behalf of the citizens.
There were other characters that I was offended by- Elrond, who came off as a selfish old man; Frodo, who NEVER turned on Sam until right near the end of the third novel; the Ents and their 'hasty' behaviour- but there is a 1000 word limit on this, so I shall move on to.
Cheap humour! Yes, we all know it was there! In fact, the majority of the people probably LAUGHED at Gimli 'chatting up' Eowyn (which stripped him of his dignity) the 'dwarf tossing' jokes, Gimli hitting an Orc in the testicles, or even- God forbid- poor, butchered Legolas catering to the easily amused masses with his 'shield-boarding' stunt.
The Two Towers was NOT a humorous book! Sure, the series did have minor, humorous moment, but those were tasteful, and fit into the flow of the story. They did NOT make the book seem cheap. Overall, Lord Of the Rings is a very dark fantasy- it is one of literature's greatest fights between good and evil, and it WORKS as a dark fantasy.
Incidentally, anyone who says that Legolas' stunt merely proved how well Elves can balance shall earn my eternal scorn. If you want an example of the talents of Elves, either read the book, or admire Legolas' way of mounting the horse earlier in the movie. That was well done, and in character.
And taking into account the word limit, I shall bring up my last major grievance- all that was cut from the movie, and the add-ons.

The second book had one of the best endings I have ever come across in a novel. It moved me to tears, and had me racing for the third book. Peter Jackson cutting Shelob and a pass of Cirith Ungol left me distraught. I don't care if they will be in the third movie- the simple fact of the matter is, these moments were in the second book, and last I checked, the movie being screened WAS The Two Towers, an adaptation of the second book.
Arwen. Ah, she annoyed me in the first movie, taking Glorfindel's place and in the second movie, she made me want to throttle something. If you go to the appendices of 'Return of the King', and find all the places she was mentioned in the trilogy, you will notice that her name never even appeared in the second novel. So why has she been given such a significant part in the film?
Damned political correctness.
Jackson was adapting a classic. It's a classic because people like it the way it is. If you are going to adapt a classic, then do it right, and do it the way it was intended- that's how the books got popular, and that's how they should remain as a film.
E-mail me if you wish to contend my views. I've put my address in for a reason, and I'd be happy to debate the issue further.