Hmm, tough one. I think Christ Child's Lullaby is my favorite. I've got two versions on CD, one in English by Kathy Mattea and one in Gaelic on a Green Linnet Christmas compilation. I also like Father Christmas by The Kinks and Christmas at Ground Zero by Weird Al, when I'm in the mood for something less than serious.

As for most hated: pick any of the modern crap being played in most malls. Santa Baby, Mery Christmas Baby, etc.