Depends on what you are interested in.
Christmas is very busy, cooler, dryer and incredible fun with the holiday festivities across property. After New Years Day the crowds leave and the hours are shorter. We prefer early December to get smaller crowds and the holiday festivities.
Easter is another very high crowd period with warm, but usually still very nice weather. You are still in the dry season. Flower and garden at Epcot is in full swing as are the spring concerts. Once you get past the week after Easter, crowds drop off.
End of August into September is hot, humid, afternoon/early evening rain and the peak hurricane season. Crowds are less then summer normal in late August and even really slow after Labor Day.