I voted for December - specifically, late December after the Christmas entertainment stops being an after-hours upcharge in the MK and becomes part of the entertainment package of a normal operating day (and thus included with your Annual Pass). And I LOVE the nice, long, operating days in late December at the MK with midnight and 1 AM closings and the night parade running twice nightly! With Holiday Wishes, the Castle lighting show, Celebrate the Season Castle show, and of course the Christmas parade, this is HANDS DOWN my favorite time to be in the MK and WDW! The extra crowds don't bother me one bit, because with the longer operating day combined with Fastpass, I can still get on more attractions per day during peak season than I can with the ridiculously early closing time of 8 PM during slow season (when the night parade only runs a scant 2 or 3 nights a week, one time a night)
Another reason I LOVE late December (and winter and general) is the weather is nice and cool (well, as cool as it will likely get in Florida - I prefer my temps between 50 and 70, and winter gives the best chance of this).
At the other parks, I enjoy the longer hours at those as well. I LOVE those nice 8 PM closings at Animal Kingdom and the sun goes down early enough in winter to get a ton of NIGHT RIDES on Everest, which is a rarity at that park during most of the year. The 8 PM close at AK also gives me a nice, 12 hour full day at that park so I don't feel rushed and have time to savor all the self-guided animal trails while still riding all the rides and seeing all the shows. Plus, the park is running the Jingle Jungle Parade instead of the Jammin' Jungle Parade!
At the Studios, in addition to the Osborne Lights, you get the nice, long, operating days, and Fantasmic! for 2 shows a night, 7 nights a week, instead of just 2 nights a week in the slow season.
My "runner-up" months would be March and April. Again, I enjoy the extended hours and the more satisfying visits these give me at the parks. The Christmas decorations, trees, and music loops are gone, but Epcot offers my second favorite WDW season with its Flower and Garden Festival and all the topiaries.
And I enjoy the MK on Easter Sunday, with the special Main Street Easter music loop, and the Easter Bunny makes an appearance in the afternoon parade.
The only negative about March/April is the weather is more likely than winter to be over 80 during the day and thus too hot for my taste, and I may find myself having to constantly drink ice water and pour it down my shirt to keep cool. But this is not nearly as bad as the summer where temps over 90 is the norm, even the nights are humid, sticky, and miserable, and the thunderstorms seem to come nearly every day!
