I love O Holy Night and Lo How A Rose Ere Blooming for traditional hymns, as well as the Holly and the Ivy and In Dulci Jubilo.
For more modern songs, I really enjoy the Christmas Song, as sung by Nat King Cole, mostly because I love his phrasing. I also really love a good concert band or orchestral version of Sleigh Ride, mostly because I find the percussion part so awesome. As far as rock bands doing Christmas songs--I have to vote for U2's Baby Please Come Home because well, I love U2.
Truly, though, there is very little Christmas music I dislike, as I was in choir all through school, and Christmas music is our bread and butter. I know more hymns, carols, and whatnot than the average person because of it, but I think that's awesome.
A great reference:
http://christmassongbook.net/