Sometimes it's hard to declare absolute favorites. Favorite movie, favorite book, favorite food - I'd really have to give you several different answers within separate sub-genres.
Until a week or two ago, I would also have given you three or four different songs as my favorite Christmas song. But now I have a hands-down winner, my absolute favorite Christmas song, ever, with no distinction of category: O Holy Night, by a small, little-known group you might have heard of called The Mormon Tabernacle Choir.
This particular rendition of O Holy Night was arranged by Mack Wilberg, my personal musical arrangement hero. Forget everything you ever thought about MoTab being bland and overly grandiose with an obtuse sound. With the direction they've had in recent years, their music is surprisingly subtle and nuanced.
I wish I could embed this video for you, but I can only give you the link. Please click on it and give it a few listens. It takes more than one to see the genius, but all the best songs need multiple exposures to be fully appreciated. The really good part starts at about 3:47. My second-favorite moment is at 5:08ish and my absolute favorite is at 5:28ish.
Here is the video. Buy it on iTunes if you like it enough so you can listen to a better-quality version.