The only two commercially made Christmas beers I truly enjoy and will go out of my way to stock up on are Celebration and Samichlaus. There are a few others I find "pretty good", but these two I've mentioned are the only ones I find to be as satisfying as a good homebrewed one.
Regarding Samichlaus, one year I found a bottle I had unintentionally tucked away...it was in a corner of a basement cupboard for more than 7 years (and possibly as much as 10). It was in remarkable shape and smooth as silk. I don't think I would intentionally buy Samiclaus to cellar it since I enjoy it quite a bit in its original bottling year, but it was interesting to discover how well a filtered strong beer can hold up after so many years.
I've got Celebration from the late '90s and Sammy from 2000 +.
It's a lot of fun doing a vertical tasting seeing how the recipe has changed, and when the accountants got ahold of it.