I chafe at people who insist Die Hard is a Christmas movie because they're often doing it 1) out of irony, because beyond its Christmas iconography, it's not very festive or Christmas-y, and the Christmas aspects are meant to darkly and comedically oppose the movie's violent nihilism as well as the protagonist's hard-as-nails wise-assery, and 2) to be contrary; any conversation I have about this matter involves the pro-side argument being "Joy to the World plays," or "I have a machine gun ho ho ho", and a knowing smirk.
I do think plenty of people do watch Die Hard around Christmas, which makes it a Christmas movie by habit (Royal Tenenbaums hits several quadrants for me: I first saw it with family on Christmas, it has the Christmas Time is Here song, and it being set in NY in winter is very Christmas-y) for those people. But looking at it purely as a movie with Christmas stuff, I'd put so many others (Shane Black's entire filmography) ahead of it.
For the longest time, the UK The Office was my Christmastime watch, simply because I first saw it when my sister came home from college for Christmas and put it on BBC America. So maybe I'm not the best judge of this.
In terms of Christmas-genre movies, my tops are Christmas Story, Muppet Christmas Carol, and Christmas Vacation.