Let's see...I've been in September, October/November, December and January (I avoided it like a plague in the summer time) September is still pretty hot. We were there two years ago and temps were in the upper 80s and low 90s I believe. The one time we went in December (it was the second week of December so the crowds were still low) they were having an unusually warm week or something and it was still hot. Of course when we went in mid-January it was nice...cold at night though, but nice during the day. No lines at all. That was back in 1999.
The best time we've ever been is for Halloween. We get there on the 31st of October and get to go to the Halloween party and stay through the first week of November. It's still warm enough during the day to go swimming but it's not like insanely hot. The lines are very minimal. You usually don't have to wait more than 10 or 15 minutes in lines. October 31st is the first day of the value season too, so that's why we show up on that day. I highly reccomend that time of the year to go.