Over the past 16 years we have been July, August, with the kids who are now all grown up. It was horrendous with long lines and stifling heat, but we had no choice as we had to go during the UK school holidays.
Once the kids had left the nest we tried Mid September ( Quiet but we got a hurricane)

Early December, quiet with nice weather. March, the parks were quiet but the weather was terrible, it was Daytona bike week and I have never seen so many cold bedraggled bikers (49degF at noon) and our favourite early May which normally has short lines and good weather.
Roll on May 2nd to 18th.