I like heat and humidity, so I won't comment on going in the summer.
For part of our honeymoon, we hit a couple of amusement parks, and we went in June but before school got out, and on weekdays. It wasn't deserted, but it was much, much better than it would get in a few weeks. It might make less of a difference at Disney, because it's so national (international) that *some* schools are bound to be out, and people might take their kids out early to go. Might be worth a try.
Personally, I love Epcot. Always have. Busch Gardens is fun, but we have a more local one, so we don't usually bother going to the Florida one when we're there. I went to MGM years ago, didn't like it much, but I'm sure they've added stuff since. The Animal Kingdom sounds fun, but I've never been there. Universal Studios is fun, but as I recall a lot of the rides are best if you're a bit older, maybe a teenager. Again, though, haven't been there for some years.
Do rides that are likely to have long lines first thing in the morning. Splash Mountain, or other roller coasters, the most popular rides. You'll hit them with the shortest lines if you do them first thing, and that way you can get one or two out of the way before the lines get too long. You might also consider starting at the furthest end of the park and working your way back (most people start near the entrance and work their way out).
