You guys nailed all of my favorites:
It's a Wonderful Life
The Madagascar Penguins Christmas Caper is pretty funny, although it's only about 5 minutes long.
All the old shows are great, too (Grinch, Rudolph, Santa is Coming to Town, etc...). It's fun with my 2 1/2 year old daughter getting old enough to start liking those. We watched Rudolph the other night.