If you go by what a guide book says, I'd go with the Unofficial Guide to Walt Disney World. Their info best matches my experiences.
Every source will look at a year as a very broad sample and should be taken as an estimate. A Guest should plan their trip around the following factors:
- size of group
- mobility of group
- attractions of interest
- park hours
- water parks a must?
- day of the week vs. park
- special events (food and garden festival)
- extra magic hour
- weather you like (cool, warm, hot, hot and humid)
This is assuming work and school permit your travels. The months and weeks of lowest crowds are different than best time to visit (IMO).
If you don't want to be cold, like going to the water parks, like minimal lines, don't mind some humidity, don't mind shorter park hour... Early May is my suggestion. One of my favorite times of the year!