Hmm...
1. Delibird, for additional delivery services.
2. Stantler, to pull the sleigh. I know in the anime Christmas episodes back in the day he used either Ponyta or Rapidash, but I think that was only because Stantler hadn't been revealed yet.
3. Jynx. Santa is actually a Jynx, y'know.
4. Fletchling. Robins are very Christmassy birds, and Fletchling is based on the Japanese Robin, so it'd make sense if he had one perched on his shoulder.
5. Hypno. Because let's face it, the idea of an elderly man coming down the chimney in the middle of the night and sneaking into a child's room couldn't possibly be any creepier than it already is. Hypno would be for sending children still awake back to sleep.
6. Beartic. For when all else fails, if Santa needs to make a quick getaway from child protection services, or face the wrath of a spoiled brat, or fight off thieves. Good to have a terrifying angry bear with a winter theme.