I voted for May.
Now that we have school age kids, we are somewhat limited to summer, christmas break, and spring break.
Before kids we enjoyed the first week to 10 days of May. The weather wasn't too hot yet, and spring break was over so crowds are low. I also liked the first week of October, for the same reasons - although you do have a slight chance of weather (hurricane) problems at that time.
I could certainly see early December's appeal - assuming you don't want to use the swimming pools alot.