We were there on July 4th in 2000 and 2003 and it was definitely the busiest day of both trips. The first time we went to Epcot where we were able to see the MK fireworks before the Epcot ones, which was a bonus, but the crowds were a bit painful, particularly at going-home time. MK was full that year, and closed to new entrants, so you can imagine the queues. Last time we went to Sea World, hoping for more space, and found that much more pleasant, as the crowds didn't seem as overpowering and there is much more variety in what you can do during what is a very long day. Also, you can get a seat in the stadium to watch the fireworks if you go an hour or so before. I would have to say that of all the fireworks displays we've seen at all the parks (each several times over), the one that sticks in our mind as being the most impressive is definitely Epcot on July 4th, crowds or not. My feeling about the MK fireworks display is that unless you queue for about two hours beforehand on Main Street or up on the railway station looking down it, then you're never going to get the sort of view you'd hope for. We've gone four or five times and have never got a particularly good view yet, whereas at Epcot the area is much more open so you've got a much better chance of a decent view.
