Well I guess that song slowly, slowly has grown to be somewhat enjoyable to me (still wish he'd redo it with old equipment).
Considering how many classics McCartney has written and considering what a sentimental, family-orientated guy he is, you would've thought he would have come up with the Christmas classic. But I give the nod to Lennon here. Lennon, with the Phil Spector effects and mandolin and choir and all that really nailed it for my money. "Wonderful Christmas Time" always sounded like McCartney was stoned on pot and noodling around on a cheesey casio keyboard or something.
But yeah, those Merry Kringle fan-club singles they sent out every Christmas beats both of them.