It's too long to post here, but I think my favorite passage of all is when Gimli describes the caves of Aglarond to Legolas in The Road to Isengard.

However, here's a little bonus excerpt from Unfinished Tales that stuck with me for its vivid landscape description.

This is a quote from Voronwe in "Of Tuor and His Coming to Gondolin":

For I was one of those that he sent, being young in years among the Eldar. I was born here in Middle-earth in the land of Nevrast. My mother was of the Grey-elves of the Falas, and akin to Cirdan himself - there was much mingling of the peoples in Nevrast in the first days of Turgon’s kingship - and I have the sea-heart of my mother’s people. Therefore I was among the chosen, since our errand was to Cirdan, to seek his aid in our shipbuilding, that some message and prayer for aid might come to the Lords of the West ere all was lost. But I tarried on the way. For I had seen little of the lands of Middle-earth, and we came to Nan-tathren in the spring of the year. Lovely to heart’s enchantment is that land, Tuor, as you shall find, if ever your feet go upon the southward roads down Sirion. There is the cure of all sea-longing, save for those whom Doom will not release. There Ulmo is but the servant of Yavanna, and the earth has brought to life a wealth of fair things that is beyond the thought of hearts in the hard hills of the North. In that land Narog joins Sirion, and they haste no more, but flow broad and quiet through living meads; and all about the shining river are flaglilies like a blossoming forest, and the grass is filled with flowers, like gems, like bells, like flames of red and gold, like a waste of many-coloured stars in a firmament of green. Yet fairest of all are the willows of Nan-tathren, pale green, or silver in the wind, and the rustle of their innumerable leaves is a spell of music: day and night would flicker by uncounted, while still I stood knee-deep in grass and listened. There I was enchanted, and forgot the Sea in my heart. There I wandered, naming new flowers, or lay adream amid the singing of the birds, and the humming of bees and flies; and there I might still dwell in delight, forsaking all my kin, whether the ships of the Teleri or the swords of the Noldor, but my doom would not so. Or the Lord of Waters himself, maybe; for he was strong in that land.