Seems we're getting our Christmas movies confused with our Christmas specials, but oh well.
Merry Christmas Charlie Brown is the most sincere and has held up the best. I can go without seeing everything else, but if I miss this, it's not Christmas. BTW, it was kind of shady of ABC to run this on Tuesday (yes, you already missed it). The quintissential Christmas program shunted to late November? Shame.
Nightmare Before Christmas is my favorite "movie" of the season, but it really spans two holidays, so it kind of gets an asterisk. Nevertheless, like Charlie Brown, it is timeless and sincere and I expect I'll love it as much 30 years from now as I did when I first saw it.
Sentimental favorite: Emmet Otter's Jug Band Christmas from 1977. Perenially overlooked and with songs by Paul Williams (who also wrote "The Rainbow Connection" for The Muppet Movie), it's easy to love this obscure but heartfelt Christmas story. Jim Henson at his best. Seek it out on DVD if you haven't seen it.