There are too many to choose from! But one of my favourites has to be Merry's description of the Ents at Isengard.

First,

'Let me see,' said Merry: 'five nights ago-now we come to a part >of the story you know nothing about. We met Treebeard that >morning after the battle; and that night we were at Wellinghall, >one of his ent-houses. The next morning we went to Entmoot, a >gathering of Ents, that is, and the queerest thing I have ever seen >in my life. It lasted all that day and the next; and we spent the >nights with an Ent called Quickbeam. And then late in the >afternoon in the third day of their moot, the Ents suddenly blew >up. It was amazing. The Forest had felt as tense as if a >thunderstorm was brewing inside it: then all at once it exploded. I >wish you could have heard their song as they marched.'

'If Saruman had heard it, he would be a hundred miles away by >now, even if he had had to run on his own legs,' said Pippin.

'Though Isengard be strong and hard, as cold as stone and bare >as bone,

We go, we go, we go to war, to hew the stone and break the door!

There was very much more. A great deal of the song had no >words, and was like a music of horns and drums. It was very >exciting. But I thought it was only marching music and no more, >just a song - until I got here. I know better now.'

Then,

As soon as Saruman had sent off all his army, our turn came. >Treebeard put us down, and went up to the gates, and began >hammering on the doors, and calling for Saruman. There was no >answer, except arrows and stones from the walls. But arrows are >no use against Ents. They hurt them, of course, and infuriate >them: like stinging flies. But an Ent can be stuck as full of >orc-arrows as a pin-cushion, and take no serious harm. They >cannot be poisoned, for one thing; and their skin seems to be >very thick, and tougher than bark. It takes a very heavy >axe-stroke to wound them seriously. They don't like axes. But >there would have to be a great many axe-men to one Ent: a man >that hacks once at an Ent never gets a chance of a second blow. A >punch from an Ent-fist crumples up iron like thin tin.

'When Treebeard had got a few arrows in him, he began to warm >up, to get positively "hasty", as he would say. He let out a great >hoom-hom, and a dozen more Ents came striding up. An angry >Ent is terrifying. Their fingers, and their toes, just freeze on to >rock; and they tear it up like bread-crust. It was like watching the >work of great tree-roots in a hundred years, all packed into a few >moments.

'They pushed, pulled, tore, shook, and hammered; and >clang-bang, crash-crack, in five minutes they had these huge >gates just lying in ruin; and some were already beginning to eat >into the walls, like rabbits in a sand-pit. I don't know what >Saruman thought was happening; but anyway he did not know >how to deal with it.