The combination of cooler (but still warm to us!) weather, smaller crowds and Halloween celebrations mean we have a great time.
I'd love to try a visit at Xmas, but the crowds are my first consideration, after the weather, and early Oct is a great compromise.
When we were in SoCal October 9-14. The hotels were packed and Disneyland from Friday evening to Monday night was crammed with people. While in line for Indiana Jones I found out that Utah and Arizona schools had those 2 weeks off for fall break too. Next year we will try early November :)
My new favorite time, however, is in late April/early May. Beautiful weather, empty parks.
June 2005 - Week in OrlandoHot weather, afternon showers, decent crowds at Disney and Universal. Perfect for water rides, average waits, and good swimming.
September 2007 - Week in OrlandoHot weather, big storms here and there, zero crowds at parks, rode everything 3-4 times. Good swimming weather.
April 2008 - Week in Myrtle BeachWeather was late 60's to 70's. Not quite swimming weather but warm enough to get in. Perfect for boardwalks, outdoor malls, walks along the beach, and sight seeing.
I would say that June is my favorite out of all of them. but the other two worked fine. So probably June, Sept, and April in that order as my faovrites.
We've gone in August and November before and the crowds seem to be the same.