THANK YOU! I knew someone would come up with the goods!
I suspected it was The Two Towers since that was my favorite and the scene was pretty good as I recall.. but it's all pretty much a blur otherwise.
I am very impressed that at the level of geekiness on this board which had my question answered in THREE MINUTES.
I can tell you some trivia about it although I'm not geeky. They shot the whole sequence at night, often with big hoses making rain on the cast, and it took months, so it was the most universally hated sequence in the trilogy from the actors' POV, not least because there were virtually no women on set for all of that time. At one point the Uruk Hai (or whatever) who were played by mostly Maori extras, had been standing in the cold for hours while the set was made ready. To keep their spirits up, they started singing Maori songs, which I'm sure was recorded because it was so weird seeing a bunch of orc monsters singing Pokarekare Ana.