Difficult to choose - so many films that we re-watch...
Home alone 1 or 2 always enjoyable - ds2 has managed to see them at least 3 times each on tv this year (not Christmas time!) and is begging me to get him the dvds for a christmas present... He sits there roaring with laughter and it's infectious - lovely to hear.
Muppet Christmas Carol, Nightmare before Christmas, Nativity 1 & 2 (hopefully we'll see 3 this year!), Elf, Arthur Christmas, The Snowman, the Snowman and Snowdog, Miracle on 34th Street - lots of familiar films that keep cropping up on everybody's lists - but for good reason - they're great!
Sound of Music and Mary Poppins both need to be seen at Christmas too, even though they're not really set at Christmas time, they've come to be associated with Christmas.
As outliers - there are a couple that spring to mind...
Rise of the Guardians - I wasn't keen on watching this tbh, judging the dvd by its cover! However, was soon converted - it wasn't at all what I thought it was going to be from the cover, and it's a great animated film, beautifully done graphics and a genuine adventure that's not saccharine sweet, There's also the Easter Bunny, Jack Frost, the Sandman and the Tooth Fairy involved... DC probably saw it when they were about 5 or 6 at the youngest, not sure I'd go much younger than that, not least because Santa is a grumpy old Russian St Nick rather than the coca cola all american Santa you usually find in films (but none the worse for that!).
The other one is Tom and Jerry's Nutcracker Tale. The Nutcracker is a very traditional Christmas tale - and the Tom and Jerry version is great - not least because it is mostly to the Tchaikovsky ballet music with very little talking in - so if the dc want a film on and you don't it works as a compromise... They get to hear nice music without realizing, and there are lots of very funny moments as only T&J do - plus it's the sort of film that you can dip in and out of without feeling you have missed out on too much (although it's nice to watch through too!). DC will usually put it on several times over christmas, often if they're playing a game but still want the tv on or because they want music on but can't agree on what to listen to. Definitely a dvd to have in the collection and use lots 