I've got a lot of love for Slade and Shakey and it never feels like christmas until I've heard Fairytale of New York but I'll put a vote in for Back Door Santa by Clarence Carter.
https://youtu.be/s0NoalRsk5w
For the worst, until this year I'd have said the Millennium Prayer by Cliff or Stay Another Day by East 17, but after hearing Sam Smith's nasal falsetto version of Have Yourself a Merry Little Christmas the other day that has now topped my list. Truly awful and I haven't got the taste of sick out of my mouth yet!