The "Four Seasons" of Orlando This is not an exact Science, but this will give you a good idea.
Value Season: January, February, September, October & November This is the best time to go. The crowds are less, lodging cost less and is more available, but do realize that this is also the time that parks will do their yearly maintenance and a few attractions/shows may not be available.
Regular Season: May, June & First two weeks of December This is a great time to go. Everything is typically operational, the weather is pleasant and the crowds have not gotten bad.
Peak Season: March & April (AKA Spring Break), July & August (AKA Summer Break) While this is "Peak" Season, and there will be lots of people in Orlando, it is not without opportunity. The downside is traffic is heavy, so careful planning is very important. The upside is, this is some of the best time to go to the water parks since they are ALL open, typically all the rides are open and there seems to be a certain excitement in the parks.
Holiday Season: The last two weeks in December Orlando really does a awesome job in the department of the Winter Holidays. As much as I complain about this time of year in Orlando, it is so much fun. There are celebrations all around Orlando, that are free to the public that even includes the cold white stuff to play in. If you are only going one time in your life, I would make this the time of year.