I've got far too many to narrow down to just one, so I'm gonna be lame and do a top ten (which is gonna be hard enough!).
In no order...
Steeleye Span - 'Gaudete'
Wham! - 'Last Christmas'
Jethro Tull - 'Ring Out Solstice Bells'
Greg Lake - 'I Beleive in Father Christmas'
Blackmore's Night - 'Christmas Eve'
Bing Crosby and David Bowie - 'Peace on Earth / Little Drummer Boy'
The Waitresses - 'Christmas Wrapping'
Mariah Carey - 'All I Want for Christmas is You'
Mediaeval Baebes - 'In Dulce Jubilo'
City of London Sinfonia - 'Adam Lay Ybounden'
Ask me this again tomorrow and chances are it'd be different. I love far too many to choose a final top ten.