Without a doubt mine is "Hark the Herald Angels Sing"!! The whole message of Christmas is right there in that song, at least for me personally. Everyone has their own traditions, but mine is very much on the nativity, and it makes me cry just about every time I hear it.
I do love all the others mentioned here as well, and there are many more yet to come. I can't wait to see what is posted and how people may celebrate...or not celebrate;-)
Fun thread!