I'm fond of "Do You Hear What I Hear?" - unfortunately modern versions tend to cut out either the first verse or the last one.
Others on my "Favorites" list include:
"O Come, O Come, Emmanuel"
"O Little Town of Bethlehem" (Nat "King" Cole version)
"Let All Mortal Flesh Keep Silent"
"Carol of the Bells" (the TSO instrumental version is very nice)
"Sleigh Ride" (either of the instrumental versions - Boston Pops or Leroy Anderson)