I mean, it has to be “Grandma Got Run Over By A Reindeer” right? What else is even close?
“Wonderful Christmastime” by Paul McCartney, followed closely by “Do They Know It's Christmastime At All?”. The McCartney song in particular is just so LAZY and bland.
The Macca one really stings knowing that Lennon managed to write one of the greatest Christmas songs of all time in “Happy Christmas” and that's all Paul had to offer the world.
As for the best, I'm partial to “Father Christmas” and most of the Elvis catalog, specifically “Merry Christmas Baby” and “Santa Claus is Back in Town”. Because Elvis.