I can't really choose a single one, there are so many good quotes. Just on the spur of the moment though, there are a few that come to mind. My favorite sentiment that I think is just rather beautiful is:
Theoden in TTT, The Road to Isengard wrote:
'Yet also I should be sad. For however the fortune of war shall go, may it not so end that much that was fair and wonderful shall pass for ever out of Middle-earth?'
My favorite descriptive passage that literally takes my breath away is:
TTT, The Riders of Rohan wrote:
At the bottom they came with a strange suddenness on the grass of Rohan. It swelled like a green sea up to the very foot of the Emyn Muil. The falling stream vanished into a deep growth of cresses and water-plants, and they could hear it tinkling away in green tunnels, down long gentle slopes towards the fens of Entwash Vale far away.
I like Rohan, can you tell?

But there is also the obvious passage that is just amazing:
Sam, in ROTK, The Grey Havens wrote:
'Well, I'm back'
A wonderful end to a wonderful book.
