Nearly impossible to pick just one. The prime candidates at the moment, though, are (list subject to change without notice; offer void where prohibited by law):

O Holy Night
Christmas in the Trenches (John McCutcheon)
A Fairytale of New York (Pogues w/ Kirsty MacColl)
Lo, How a Rose E'er Blooming
Once In Royal David's City
O Come Emmanuel
Nothing But A Child (Steve Earle/Maria McKee)
Christmas Must Be Tonight (The Band)
Sound Over All Waters (Paul Winter Consort/Susan Osborn)
Much of "A Muppet Christmas Carol"
I Wonder As I Wander
In the Bleak Midwinter
Christmas For Cowboys (John Denver; don't know who wrote it)
The First Noel
Winter Wonderland
Let It Snow