There is NO WAY I can pick just one, so here are my favorites:
My favorite traditional:
God Rest Ye Merry Gentlemen
Good King Wenceles
Handel's Messiah
Huron Carol
Oh Holy Night
The First Noel (in college, we sang this and it was set to Pachelbel's Canon and it sounded so beautiful)
What Child Is This
(In college we also sang a medeley of Bring A Torch, Jeneatte Isabella, Ding Dong Merrily on High and Come Shepherds All)
My favorite contemporary:
All I Want For Christmas Is You
Santa Baby
You're A Mean One, Mister Grinch
Least favorite:
Little Drummer Boy
The 12 Days Of Christmas