The Fifteenth - O Come all Ye Faithful
...it's so full of joy and richness!
(see below for a note from Andrew about 'the chord'...)
Here's an alternative version...
This nomination reminded me of some internet chatter I'd heard last year, which focuses on a chord used in the last verse which is traditionally only sung on Christmas Day, or as the last carol at Midnight Mass. As we sing "Word of the Father," symbolic of the word made flesh in the first verse of John's Gospel, the chord on "word" is treasured in choirs all over the world, with the power to bring goosebumps and raise a smile. It's even printed on T-Shirts here and there is an article about it here: New York Times