A super easy one: Aragorn riding Brego, up on the hill, while the camera pans around with the most marvelous view ever, ending on Helm's Deep. I wait and wait and wait for that one moment. It is the entire movie to me. It is no coincidence whatsoever that that moment also has my favorite music in the entire movie as well.
Other favorite moments: the ents going to war. Not coincidentally, probably my second favorite part of the Two Towers score.
Also, the long shot of Gandalf and the Balrog plummeting into the giant cavern, because the shot is exquisite, the way the darkness brightens, the lake picks up the reflection, and then they appear, falling. Love love love it!
As for favorite line. Hm. "They're taking the hobbits to Isengard!" LOL. Or "Not listening!"