My all time favorite is 'White Christmas' and I don't think anyone sings it better than Bing Crosby (ps 'White Christmas' is also my favorite Christmas movie)
http://www.youtube.com/watch?v=tRYPWlyU_Zk
My favorite modern day Christmas song is 'The Gift' by Aselin Debison
http://www.youtube.com/watch?v=EpgpkSxO950&feature=related
My favorite fun Christmas song is 'I Want A Hippopotamous For Christmas' :)
http://www.youtube.com/watch?v=qvEX_ASH0ws
Nothin like a Christmas song to get you into the holiday season :)