Carol of the Bells is probably my all-time favorite Christmas song just...well because. I spent a good portion of my childhood wondering what that song was called and was very pleased when I found out.
Other than that, I like most Christmas music. And the strange thing is, even though I'm not particularly religious I tend to lean towards the more spiritual songs, Silent Night, We Three Kings, etc. I just think most of them are really beautiful. And also, Judy Garland performing Have Yourself a Merry Little Christmas; that song usually makes me cry, haha.