My favourites are The Holly and the Ivy and the Cherry Tree Carol . I also like the wassail ones if they can be included here - particularly the Wassail All Over the Town
I always get a good laugh out of Adeste Fidelis as crooned by Bing Crosby and the strange lyrics of The First Noel - all part of the Christmas spirit.

The one I find most annoying is the one by either Dire Straights or Chris Rhea or some shitty cross between the two its called , Travelling Home for Christmas.