^Sufjan, for sure, although my favourite stuff from his first collection aren't even the Christmas hymns (Come Thou Fount, Holy Holy Holy).
I grew up hearing the Carpenter's Christmas album, so it's not Christmas until that one is playing.
The Charlie Brown Christmas album is another must.
Although there isn't much that I'd go for in contemporary music in this area, I'm also partial to Over the Rhine's Christmas album, "Snow Angels". Their twist on O Little Town of Bethlehem is great, as is their instrumental tribute
to the Charlie Brown album (called "Goodbye Charles"). Unfortunately, neither of those two tracks are on YouTube, but the latter was uploaded for free by the band at the link given.