My favorite Christmas movie as a kid was Jingle All the Way. Yes, I know, it's extremely silly, but there was something about the quest to obtain the most popular toy on Christmas Eve that I really loved. Maybe that's because I know my parents did such a thing on occasion, or maybe it's because I was so entranced by the thought of getting my own Turbo Man figure. Whatever the case, Jingle All the Way is a movie I can't help but watch anytime it's on during the holidays.
As far as my favorite adult Christmas movie that would have to be Kiss Kiss Bang Bang. It's not a particularly Christmas-y movie, but it's set around the holidays just like all of Shane Black's films. It's also hilarious.