"Best" is hard to define. What are you looking for? How old are your kids?
July is going to get you all the fireworks, parades, etc. But the park will be packed and it will be hot. So you'll need to plan for that. If you have little kids you'll need to plan for naps, etc. You'll need to make sure you have a fastpass runner.
September is cooler, but you'll only get fireworks on the weekends. Park hours will be shorter during the week. You may have closures during the week for holidays overlays for Space Mountain & Haunted Mansion.
In Oct they'll also start closing stuff for Christmas decorations.
Nov/Dec you get to see all the Holiday stuff which is great, plus all the Holiday treats!
Crowds are smaller in the off seasons but park hours are shorter.
In DCA, the longer you wait to go, the more stuff is opening up. However, the bigger the crowds are going to be as well.
World of Color and Silly Symphony Swings opening up this spring.
Ariels Grotto within 12 months
Cars land within 24 months
Plus the new entrance w/ Red Car trolleys whenever they figure out how to do it.
So, we go three to four times a year!

We used to go more often, but now we live 1000 miles away!
