"Christmas in Dixie" by Alabama (GREAT song from my homestate brothers! Alabama's heyday was during my high school and college years and I love'em)
"Happy Christmas (War is Over)" by John Lennon (simply a beautiful song, despite Yoko's back-up vocals)
"I Believe in Father Christmas" by Greg Lake (one big, majestic Christmas song!)
I believe those are my favorite three Christmas songs that ALWAYS get me in the mood. I also love "The 12 Days of Christmas" by Bob and Doug McKenzie.
Yep, I know that four songs not one song. Oh well.