I have to admit, I was stumped for an answer - you see, there's so many.
There's joyful ones like "Ding Dong Merrily On High", "Joy To The World" and "Hark The Herald Angels Sing".
There's reflective ones like "In The Bleak Mid Winter", or "As With Gladness"
Quirky ones like the Calypso Carol.
Ones that focus on some of the characters in the Christmas story like "We Three Kings" or "While Shepherds Watched".
Some carols I associate with school like "Away In A Manger" or "Infant Holy, Infant Lowly"
So hard to pick a favourite.
They all mean different things, have different associations, say different things to different people.
I can't pick a favourite, but I suppose that's OK - I just look forward to singing them each year.