I usually go in Sept (hot, but low crowds), mid Oct (warm, but a little more crowded), first or second week of Dec (weather is iffy, but crowds are great), and first or second week of May (perfect time for fairly low crowds and great weather). I've also been a couple of times in Jan but would not do that again (cold and too many Brazillian tour groups).
I went this year the weekend after Thanksgiving and could not believe the low crowds and fantastic weather. I was there Nov 26th-Dec 4th and have never seen WDW as empty as it was. Universal was also dead that week. It was great!