My favorite Christmas movie to watch is Holiday Inn
starring Bing Crosby and Fred Astaire. It isn't particularly Christmas-y though it is themed around holidays. It is funny as heck and has great music (White Christmas; Easter Parade; Happy Holidays; Be Careful, It's my Heart; and the same big band song throughout for different numbers).
It is a predecessor of the lame movie White Christmas. Fred Astaire dances while lighting firecrackers. The lead men fall in and out of love very easy. There is a pretty politically incorrect number about Abe Lincoln and there is inappropriate comic relief with Mamie and her kids.
A great synopsis of it is here but I recommend you just rent it or watch it when it is on TV. It doesn't get played as often as the aforementioned lame movie (though I should admit that Missy and I sing "Sisters" quite often).
Here she comes, Miss Hit and Run...
Wait - I should mention that I also adore It's a Wonderful Life and most other Frank Capra movies. Have you seen the SNL lost ending of It's a Wonderful Life? Phil Hartman as Uncle Billy just cracks me up. Well what are we waiting for? Let's go get him!