Cosmetic heRO stuff is always fun.
I personally think the tuxedo-like santa suit (and maybe someday summer event suit, etc) would be the most useful since cosmetic stuff is all about the moment and the novelty, so it'd be nice to be able to put it on for however long you want; 20 seconds or two hours, and be able to take it off when you want.
Also, when it's a re-usable tuxedo-like equippable item, you can make it really rare and make it hard to hunt/quest for. To me that's a lot more exciting than just buying it over and over again from some NPC!
But if selling the regular santa suits at an NPC is the most feasible idea in terms of coding, then I guess that's still something