Not sure if it’s appeared on this thread yet, but my favourite (forgive me) is underneath the tree by Kelly Clarkson - it has everything I love about a Christmas song.
The worst Xmas song of all time is Do They Know it’s Christmas; I can’t bear it, it offends my ears, and I have to turn it off. I watched a Paul Weller documentary some time ago and he was asked why he looked so miserable in the video, and his succinct reply was “because it was a dreadful song”. I couldn’t have agreed more.