Mine is the anticipation. Decorating the house and making it all sparkly. Shopping for presents for my nearest and dearest that I know they'll love and enjoy...
My eldest just said her favourite part of Christmas is opening her stocking and getting lots of silly things from Father Christmas that I would say no to and thinking back on all the good things she has done that year to deserve her presents. (I have never told my children that FC brings them presents if they're good so not quite sure where that comes from!)
My youngest says it's Christmas eve and all watching a film together then waking up with a stocking filled with presents on the bed.