My wife's undisputed favorite has got to be The Year Without A Santa Claus (the heat mizer/snow mizer one).

Our daughter isn't to the age of talking yet - but the way she laughs at Squidward - I'd guess that she would pick The Bikini Bottom Christmas Special.

As for me:
Animation-wise I like to watch The Grinch & Rudolph & Garfield every year.
Live-action-wise I like Santa & Pete (James Earl Jones rocks) & Home Alone (Daniel Stern made a new Christmas movie last year called "Battle Of The Bulbs") & Santa Claus The Movie (that scene where Cornelia puts out the plate of food in the snow for Joe has been burned into my brain for 25 years!)