My favorite time is Jan, but I don't mind bundling up for the parks at night, and we're from Michigan, so as long as there isn't SNOW on the ground, I'm good.
This past Sept we went and WOW it was hot but it was a "I like this" hot. Michigan summer was super tame this year and I just enjoyed the heat for a few hours at a time with some air conditioning breaks inbetween. It was a nice change of pace.
Now we're planning a 14 night trip the end of Aug, with APS and going for a week in Jan. Super excited, and it's nice to experiance the extremes in weather.