I don't know how I missed knowing of its existence until this month, but hearing Steeleye Span and Maddy Prior sing "The Boar's Head Carol" has left an indelible impression. It's worth hearing for the powerful a cappella intro alone. Apparently, it was recorded in a room with great acoustics, as the ensemble's voices seem to ping off of the walls and demand that your ears take notice.
In a version that doesn't quite work, the carol's lyrics about the boar's head dish might sound like they're asking to be parodied. This rendition, however, gives them legitimacy.