The best ones:

Fairytale of New York - The Pogues ft Kirsty MacColl (obviously)
Just Like Christmas - Low
I Believe in Father Christmas - Greg Lake
Anything off the Sufjan Stevens Christmas songs boxset

And I have a soft spot for the stuff like Slade and Wizzard. And All I Want for Christmas Is You by Mariah Carey, the only bearable thing she's ever done.