I was watching Live with Regis and Kelly the other morning and Enya performed "White is in the Winter Night" from her new album And Winter Came. Suffice it to say, I immediately downloaded the album and have been pretty much obsessed with it since. I love it. Classic Enya for Christmas. Very ethereal and just beautiful. I highly recommend it.
I also adore Josh Groban's Christmas album Noël and the song "Believe" from The Polar Express soundtrack (and pretty much the rest of that soundtrack as well).
But my absolute favorites are the classics, especially "Hark the Harold Angel Sings," "Angels We Have Heard on High," "Have a Holly Jolly Christmas," and "Sleigh Ride."