my personal favorites are the wessex carol and hairy mary- hey, it useta be a christmas carol
the seven joys of mary- i'm not sure if it's a christmas carol per se, but what a beauty, especially as sung by maddy and june
"river"- joni mitchel is damn good at melancholy, and while we're on the subject, "christmas in the trenches" has got to be among the saddest. but a favorite.
two good pagan carols from maddy prior's song cycle "year"
"red and green" and "long shadows"