At this point in life, frankly, it's a rare Christmas carol that doesn't annoy the hell out of me, unless it's done really well.
I love "O, Holy Night" if it's sufficiently dramatic and terrifying (I love a really imperiously commanding "Faaaaall on your knees!!"). I love the melancholy "Have Yourself a Merry Little Christmas," especially if done by someone really Greatest Generation-y like Judy Garland; it so perfectly illustrates the outlook of people living through a potentially world-changing war. I like "Carol of the Bells" by almost anybody. I have come to appreciate "Santa Baby." "Deck the Halls" brings back fond memorie of the time my son voluntarily memorized the three verses and sang it solo to his first-grade class.
Other than that, it has to be a REALLY good rendition for me to really genuinely like any Christmas song. Tonight while driving, my sons and I listened to a CD of Christmas songs. Unfortunately, almost all of them were bad versions.
On the other hand, I feel a certain cultural obligation to listen to them with my sons and pass along their heritage. If I don't, it's possible they'll never have any Christmas music until Kanye gets around to recording Christmas rap.
All that said, here is my vote for the worst Christmas song ever -- a song I had never heard and didn't even realize was a Christmas song until the two seasons I spent working at Macy's, where it was on the Muzak rotation.
[youtube=425,350]http://www.youtube.com/watch?v=tiDht6CcYgE[/youtube]