We go every year at Christmas, it is magical.
MK is my favorite during Christmas, walking down Main St., the Castle lights, extra shows/events and so much more.
DHS Osborne lights are fun, we visit them a couple times every year since they move from Little Rock when my daughter was much younger and smaller.
EP used to be much better but many of the countries have storytellers describing their version of Santa Claus and the Candelight Processional is a must see if you haven't done it. We used to go every year on Christmas Eve when Edward James Olmos would narrate the Christmas Story, it was truly inspiring and was chilling his last year doing it the 24th-26th.
AK is not much different during the Christmas holidays, nothing particularly special from my perspective.
The Deluxe resorts decorate heavily, the GF has a full size gingerbread house with sales of various gingerbread and chocolate items with resorts such as the Boardwalk Inn having a rather large gingerbread carrousel and other wonder decorations.
The parks are VERY busy, always long lines but knowing what you are getting into is the main battle.