Before we both lost our jobs in 2009, we used to go 3 or 4 times a year, so we've been during every season. It's really a personal preference because some people like heat and some like cold. We prefer cold. We went to WDW once in May and it was the WORST! Literally, we left the airport and my lungs seized up. The heat and humidity were too much! I had to use an inhaler and we stayed in the room for a while until I got used to it. AND....LITERALLY...it was so humid that our sheets stayed damp the entire trip! NO JOKE! We tried everything and the sheets just wouldn't dry! I would say not to go in May...or any other warm month.
When we have money now for WDW we always go in January. Make sure you don't book during the marathon because it's super busy. The second or third week is usually the time to go. It's the cheapest and least busy, not to mention that it is nice and cold!! I can't find the pictures, but I have pictures in the middle of Adventure land, on the Pirates ride, and underneath one of the gigantic Christmas trees with literally ZERO other people in the shot! I'm telling you, mid January is the time to go!
Oh yeah....late October early November for the food and wine festival at Epcot is also AMAZING. Weather is decent and the event is FANTASMIC! Warmer than January but still cool enough. The Halloween party is great too!