I think the worst is “Wonderful Christmastime” by Paul McCartney, though I have not heard this Elton John tune that I am aware of.
Well now, I could consider a tie vote. That is also one horrific Christmas tune.
Oh, yes: “A Wonderful Christmas Time” - almost forgot about that one (dammit!). A sickly sweet Yuletide confection chock full of empty calories, hung upon a stupefyingly repetitive chorus that makes you want to drive your sleigh into a brick wall. Every year, I hear that song and almost wish that Paul had been the one living in the Dakota.