Anything but the Magic Kingdom! We've done AK on Christmas Day and it was practically empty.
Epcot can be crowded, but at least it's big enough to fit a lot of people. Just don't count on doing a whole lot while you're there except for maybe people watching.
I haven't tried MGM on Christmas, and I've heard enough horror stories about MK to avoid it like the plague.
Edited to add: my biggest tip for Christmas Day is.... no matter which park you choose, get there early! Be at the gate at least 30 minutes prior to opening. So many other people take time to wake up, open presents, etc. By about noon the lines at the gates and buses are crazy.