December is no longer "slow" but I think it is still worth the visit. The MK gets pretty crowded on the days and nights that there is no party, so I plan ADR's for meals when I am in that park on non party days, however if you plan to visit on a day that there is a party you can get a lot accomplished before noontime. WE do both, a party morning, and then a later arrival on a non party day for evening entertainment.
Epcot is nice as well, but again there are Holiday celebrations throughout, so the pathways can be tight.
DHS is lovely during the Holidays, but it is a relatively small park, and gets crowded as the day progresses.
DAK is jreally nice this time of year. I find it oppressively hot during the Summer months, so the December temperatures make that park a joy.