Santa, definitely! But then I grew up on the Continent so I probably don't count DH is Irish and all the ILs call him Santy.
Only exception in our family is DS2 (2) who loves singing "Father Christmas, he got stuck, coming down the chimney, what bad luck!" at the top of his voice (well sort of, he doesn't talk that well yet, just thinks he does). Otherwise he's Santa in this house (but admittedly everyone else around here - middle England - seems to call him F.C)