A Fairytale of New York - Just the best
Christmas (Baby Come Home) - Ronnie Spector, but I like the U2 version
Blue Christmas - Elvis version is the best
I'll Be Home for Christmas - This is a classic, but it's actually preety sad. The song is about a soldier who actually won't be home for christmas
Christmas in the Trenches - Great song
Father Christmas - The Kinks, can't believe nobody's mentioned it yet.
River - Joni Mitchell, I like the k.d. lang version
Hard Times - a great old song about putting a bad year behind you. Mary Black does a great version.

I've been thinking about doing this a bit too.