I have been thinking about Christmas music a lot too. 'Tis the season. 
I like both Christmas carols and hymns as well. “O Come, O Come, Emmanuel” is so hauntingly beautiful. Definitely one of my favorites, too. “O Come All Ye Faithful” is another favorite. Newer (to me) is also “Come, Thou Long Expected Jesus.”
I’ve also been thinking about Christmas songs that I love that are not very popular (at least anymore). I’d be very curious to know what are some of your favorite non-mainstream Christmas songs. So much of what is wonderful about Christmastime to me is the nostalgia.
For example, I love the song Mary’s Boy Child (especially the out there but wonderful version by reggae/disco band Boney M.). I heard it on the radio all the time growing up, but never hear it on Christmas radio stations anymore. The lyrics are really moving:
"For a moment the world was aglow, all the bells rang out
There were tears of joy and laughter, people shouted
‘Let everyone know, there is hope for all to find peace’
Oh my Lord
You sent Your son to save us
Oh my Lord
Your very self You gave us
Oh my Lord
That sin may not enslave us
And love may reign once more
Another Christmas song I never hear anymore is “I Believe in Father Christmas” by Greg Lake. Certainly not an obviously Christian song, but I’ve always loved pondering its deeper meaning, even as a child. I’m still not sure I completely understand it, but I’m drawn to the realism yet hopefulness amidst the sometimes pollyannaish nature of many more modern Christmas songs.
One more I really love is called “Touch Hands on Christmas Morning.” It wasn’t until I heard my pastor preach a message about the verse in Isaiah that is a prophecy of how the Messiah will make all things right that I realized that was what this song was referencing when it says:
“When you gather on Christmas morning
Touch hands and thank the Lord
When you gather on Christmas morning
(Touch hands on Christmas mornin′)
Touch hands and thank the Lord
Pray that with tomorrow
The plow will bury the sword
(Touch hands on Christmas mornin’)
Touch hands and thank the Lord
When you′re trimmin’ the tree with candy
(Touch hands and thank the Lord)
When you’re trimmin′ the tree with candy
(Touch hands on Christmas mornin′)
Touch hands and thank the Lord”
The verse from Isaiah is talking about the peace that the Prince of Peace will bring and says, “He shall judge between the nations, and shall decide disputes for many peoples; and they shall beat their swords into plowshares, and their spears into pruning hooks; nation shall not lift up sword against nation, neither shall they learn war anymore.”
That image of transforming a weapon and tool for destroying and killing into a tool for cultivating and stewarding the land is actually one of my favorite Advent/Christmas images. It speaks so powerfully to our hope as Christians.
Those are a few of mine. I really love almost all Christmas music (except for very modern pop Christmas songs and the Jackson Five haha).