Wow Tim,
I can't think of a Christmas song that I don't love, especially anything by the Trans Siberian Orchestra.
But one of my favorites is "Mary's Boy Child/Oh My Lord".
You don't often hear it. It has a kind of Caribbean sound and with steel drums it’s great. My son is studying percussion performance and that includes steel drums so I tend to be a little prejudice.
If you are curious you can hear Boney M. sing it at
http://www.youtube.com/watch?v=7BVtzu59feY Dean
