A lot depends on the person. What is most important to you? Weather, crowds, price? Do you have kids? If so, are you willing to pull them out of school?
Sept is great crowd wise, but is very hot and humid. If you have kids, you probably don't want to pull them out the first week of school, hence the empty parks.
Lots of folks like Aug, but it is way too hot for me. Hotter than the sun, humid, and rainy. I thought it was crowded at the end of Aug, but others don't feel that way. Did I mention how hot it was?
Nov is awesome weather wise and crowd wise, but you have to watch for things that spike up the crowds like Veterans Day, Thanksgiving, Marathon etc. The week before Thanksgiving is just about perfect IMO.
Dec is great if you want the x-mss decorations and cp, which lots of folks love.
The spring is nice weather wise, with only moderate crowds.
So it really depends on what is important to you. Personally, I think November is just about perfect. I also like the first week of January, the parks empty out after the holidays. I also think the first two weeks of Sept are great too.