I put October, which is ironic because I've never BEEN in Oct.
My first visit in 1983 was in November (I think, it was fall and cold so November would fit.) Since then, I've been in Nov. ('98) July ('00) and May ('03).
This what I think:
January/February: Too cold.
March, April: Too hard to get airline tickets
May, June, July August, September: Too hot/crowded.
October: Perfect weather, perfect crowds (except the festival at Epcot but you can't have everything in life!)
November/December: Too cold.
As you can see, temperature and crowds are a huge factor with me. While I will go when it is cold/cool with low crowds, I will not go when it is too hot, regardless of crowds. That's why I think October is a perfect mix of good weather and low crowds.
But, as you can see from my count down clock, I'm in fact going in December. Not for lack of trying, I wanted Oct. but Dec. was the only week my BF could get time off. Plus I've never seen WDW at Christmas, so the cooler temps will be more than made up for!