So many of my favourites on here!
I love the Christmas choral music - I used to be in a small chamber choir, and really looked forward to the autumn term rehearsals, because I knew we'd be starting to practise for the Christmas concert.
Sadly I have left now - a combination of toxic choir politics (people getting really nasty about a proposed change of rehearsal venue, and biting the heads off those of us who were in favour of it), and being utterly and totally hacked off by always singing boring alto parts - I am a second alto, and in my experience, we get the notes that are left over when the other parts have got either the tune (the sopranos) or an interesting harmony line. I know what we do is vital to the richness of the harmonies, but I simply was not enjoying any of the singing I was doing, so I left.
My all-time favourite classical Christmas piece is In Dulci Jubilo - the 8 part arrangement.
I also enjoy the Lauridsen pieces mentioned by an earlier poster, as well as the Carol of the Bells, Ring out Solstice Bells, and Gaudete. And I have a very soft spot for the cheesy Christmas pop too - basically I love pretty much all the Christmas music.
Bob Chilcott has composed some nice Christmas pieces too - but our Musical Director was a bit obsessed with him, and as a result we sang rather too much Chilcott for me, and I am all Chilcotted out!