My three favorites are Someday at Christmas by Stevie Wonder, Happy X-Mass by John Lennon, and Do They Know It's Christmas - Band Aid. And I also love all Nat King Coles Christmas music.

Stevie Wonder's: One Little Christmas Tree, Bing Crosby and David Bowie: Little Drummer Boy, A Soldier's Silent Night, it is not really a song, but it makes you really appreciate the sacrifices of our armed services personnel.