Coventry Carol is lovely, but I have to say the version in the red books and the green books before that doesn't really do the carol justice. I seem to remember it should be in a different time signature than 3/4 (9/8?) and there is definitely a time change for a bar midway though. To stick it into waltz time kind of messes up the beauty of the writing IMO.
For me Who Is He (or as we say around these parts "oo isE") is just gorgeous, but again the version in the old green books was better with a euph part to die for.
There are several - we often go "off piste" when numbers and players ability allow and we often get surprised at how good some of the lesser known carols are. But we are en-debited to the SA for producing such flaming good books to play out of. Today we had a very mixed group of players - always unbalanced as we were light on cornets but with a but of sensible part allocation the group always sounded great, whether it was four players or ten.