OK. I'm amazed they haven't been mentioned but the definitive Christmas songs are Merry Xmas Everybody and I Wish It Could Be Christmas Everyday.
Trivia time, the ISO9000 consultant for a landscape consultancy I was a NED for, John Heath, lived opposite Roy Wood's mum when he was a kid and was one of the school choir on I Wish It Could Be Christmas Every Day (he doesn't get royalties).
But my favourite Christmas song is one of Jethro Tull's ample canon - first appeared in the wild on Dave Pegg's Cocktail Cowboy Goes it Alone, but there's also a version on the reissue version of Broadsword and the Beast:
Through long December nights we talk in words of rain or snow
While you, through chattering teeth, reply and curse us as you go.
Why not spare a thought this day for those who have no flame
To warm their bones at Christmas time?
Say Jack Frost and the Hooded Crow.