I think it depends on what you are looking for:
Early December can be really nice - Christmas decorations are up. But its often a little chilly - especially if you want to swim, and if what you are looking for is a BW Standard View room at seven months out you may not have much luck.
October is wonderful - warm weather, but not too hot. Food and Wine.
MNSSHP. It seems to get more crowded each year, but its managable. However, you have that
DVC popularity problem again. And Food and Wine can make getting through Epcot difficult. We like October, but I could do without Food and Wine making my vacation harder.
July is hot and crowded - but there is an energy for summer at Disney - the parks are open very late and it can be a great time to go if you prefer nights - a lot of the low crowd times you'll find the MK closed at six.
Some people swear by Christmas - too crowded for me, but again, the energy at Disney is supposed to be particularly magical at this time of year.
So pick your time and weight the pros and cons.