I voted for July. Even though March and late December are probably more crowded, the huge crowds along with the fact that it's the hottest month would make it the worst time to go. When my sister and I were in school, the only time we could go was the summer, so we always went in late August. It was definitely hot, but the crowds were never that bad.
And I can't believe that so many people picked January! You should all try it sometime. Then again, maybe not because then there will be less crowds for when I go in January!
The weather is perfect, not too hot or cold. The last time we went in January it was mid 70's, with some nights getting a little chilly, but with our sweatshirts we weren't cold at all. Rehabs aren't that bad because it's not like they close everything down. When we went it was only Buzz Lightyear and Test Track. But that wasn't a big deal because we still had Splash Mountain, RnRC, Tower of Terror, Soarin and so on. Also there's always one water park open. And the early hours really aren't that early. Animal Kingdom always closes pretty early, which is why it's always a morning park for us. Epcot is pretty much always open until 9 year round. And MGM and MK closed at around 8 when we were there, which isn't that early. The good thing about closing a little earlier though, is that it gives us time to go do other things at night....like go to Downtown Disney, Pleasure Island, other resorts, or just relaxing at our resort. And the best thing about January...the almost nonexistant crowds!!
I'm so happy we tried going in January...I would do it every year from now on if I could.