As a child of the late 60s and 70s, I love Christmas specials and while I have many on DVD, like someone else mentioned, I like to watch them when they air on TV, too. Fun knowing that a lot of other folks are watching the same show at the same time.
A more recent Christmas special I really enjoy is "Yes, Virginia." Fine animation and a nice retelling of the story.
One I remember that I'd like to see again is the animated Little Rascals Christmas Special. It's influences are Gift of the Magi and and the movie Holiday Affair with Robert Mitchum and Janet Leigh.
I've not seen it on TV in forever, but you can watch it on YouTube.