I love the 'Gloucestershire Wassail'
(Wassail, wassail all over the town,
Our toast it is white, our ale it is brown,
Our bowl it is made of the white maple tree,
With our wassailing bowl we'll drink health to thee.)

My other standards - with guitar - are 'Silent Night' (which was originally composed for guitar), 'The Twelve days of Christmas', 'The Little Drummer Boy' and for a bit of frivolous fun, Tom Lehrer's 'Christmas Carol'.
Jonathan. (UK)