I know that Silver Dollar City is not one of the top 20 parks, but their Christmas event here in the Midwest is worth noting. It is the only seasonal park that you'll find up here open in December. I only wish it got a little more attention.
As far as the major parks go, I can't comment because I have not been to them. But, please don't knock the "little guys".
My favorite specific event is the Osborne Dancing Lights at Hollywood Studios, but I voted for Magic Kingdom because unlike every other park that has one or two events, Magic Kingdom has a whole part full of events like the parade, fireworks, castle shows, snow on Main Street, free hot cocoa, lots of special character meet-and-greets, and the beautiful castle lighting....seeing MK all decorated is a sight to behold...the castle lights, the reefs along "the sky" down Main St....Magic Kingdom is the most magical of any theme park, and that magic is even brighter during the holiday season
So MK hands down