It depends
Depending on your preference for how cold/hot it may be. We have gone throughout the year, and found Dec/Jan to be the best time of year to go. For obvious reasons Dec. is a great choice, but here's another couple of reasons.
First, the weather is perfect. Great temps during the day, and just enough chill in the air at night for good snuggling back at the resort.
Secondly, the parks aren't crowded at all, unless it's during Christmas Week/New Years. Other than that, rides are walk on, shops aren't crowded, counter service goes quickly, and wait times for Resort Trans. isn't bad either.