O Holy Night - I actually prefer playing it on the piano, but hearing it sung well is breathtaking!

If you really want to get LES, try and find Elisabeth Schwartzkopf singing 'Silent Night' in the original German, with the original instrumentation (guitar and violin?). She harmonizes with herself too. I heard it once many years ago on my car radio and just sat there with tears running down my face. So simple...and extremely beautiful.