That's a tricky one. For me, ideally, I'd avoid the busiest times and the summer months. All the major US holidays see the parks mobbed (Christmas through New Year, Easter, Thanksgiving, Spring Break, President's Week, school summer holidays).
Late November/early December has the benefit of the Christmas decorations and attractions without the crowds. Mid September through mid-November (other than Thanksgiving) has the benefit of the International Food and Wine Festival and the Halloween events, plus good weather, without the crowds. April/May/June (outside of Easter and Spring break) has the benefit of great weather and low crowds. The Flower and Garden Fesitival at Epcot is pretty, but not as big a draw as some of the other events I've already mentioned.
We also enjoy early January because it's a great time to get away from the post-Christmas blues and atrocious weather here.