Either Hollywood Studios or Animal Kingdom will be your best bets. And for me and my money, Animal Kingdom will probably be the least crowded as most folks will be looking for a park that stays open later (DAK closes at 7 PM on July 4th).
By process of elimination, DAK is your best bet for the lowest crowds on July 4th.
However, if you are looking for the best fireworks... I would imagine the Magic Kingdom is the place to be. You and 80,000 other fans will have an explosive time!
Animal Kingdom will likely be the least crowded on account that its the only park with no fireworks!
I love a crowd of people from around the world, talking to them, riding rides with them, telling them about this big beautiful country of ours. But if you want to ride rides, forget it, the lines are usually 90 minutes or longer.
If you love fireworks, they put on a great show, which is set off all around the park, not just in back of the castle. They have set this show off on the 3rd as well, from what I've been told.
So if you love crowds and fireworks like I do, then go. If you love riding rides, go to one of the other parks. Oh yeah, get there early, because they close the park when it gets to capacity.
At SeaWorld the fireworks Weren't outragously amazing, but they were decent. I had to work that day and I couldn't spend the whole day in a park. I just wanted to show up and see some fireworks. I figured SeaWorld would be the easiest show to get to at the last minute, and I was right. I got there about 30 min. before the fireworks were scheduled to start, parked up front in the space of someone who had already left, walked right in, and watched the fireworks. I was completely surrounded by Brits, which I thought was pretty ironic. I was starting to think they were plotting to take back over. I sat and walked around to wait for the masses to clear out before I tried to exit, but I didn't mind hanging around because I had just arrived.
This is one of the most crowded days of the year for the Magic Kingdom. When I worked there the park would usually fill to capacity by noon. They don't really tell anyone what capacity is, and I think they change it depending on who's in charge and what attractions are open, but I'd say over 80,000 people for sure.. probably closer to 100,000. If you go to the Magic Kingdom, you have to get there early and stay there all day. If you leave, there is a good chance you won't get back in, even if you have dining reservations in the park. When the turnstiles hit that magic number, they will not let you in for any reason until a certain amount of people leave, and most people aren't going anywhere until after the fireworks. The fireworks are pretty awesome. They do a perimeter show, which means they have several launch sites set up around the perimeter of the park in addition to the normal one behind the castle. It's like the firework version of circlevision 360. I'm a pyro junkie so it gave me chills to watch. I don't know how much if any the show has changed since I saw it. I worked there 3 years ago, and then they did a modified version of the old Fantasy in the Sky show. It is set to music, but be warned, it does not have the storytelling elements of the Magic Kingdom's normal show Wishes or of Illuminations at Epcot, so if you are looking for that you may be dissapointed. It's shorter in time than most city 4th shows too. But the 360degree view makes it worth the shorter show to me.
I have no idea if Epcot fills to capacity, but Extra Magic Hours or not, it will be super crowded. I've heard they do Illuminations with an added patriotic finale. I don't really care for the current version of illuminations. There is too much time watching tiny video screens on a little globe while listening to some kind of new age/mother earth music and storyline and not enough fireworks for me. But I'm sure that they do it right for the 4th.
I've never seen the fireworks show at Disney Hollywood Studios. I've heard that the show is good, but I have no idea if it is or what the crowd is like. I've always imagined it was just a backup show for people shut out of the Magic Kingdom and Epcot. In the past they have done the "Sorcery in the Sky" fireworks and also 2 showings of Fantasmic.
I'm guessing Universal does their Cinesphere show... I saw it once on a regular park night and did not like it. They probably add alot to it for the 4th, but I hated this show when I saw it, so I don't know if they add enough to make it good.
If I go to a theme park this year, it will be to back to Sea World or to DHS, or maybe to Busch Gardens. I'm also thinking of going downtown to Lake Eola. It's free and it's a good quality show. It's probably a nightmare to find somewhere to park though.
As a dedicated Disneyphile who reads about and experiences WDW several times, I would normally advise doing the theme parks on the 4th, with all the large families from outside Florida and chanting legions of South Americans guided with adults holding pendant flags factored in. If your heart is set on doing the pyros on that day at the parks, snag a viewing spot early - that's all. If you want just straight fireworks, head on over to the Magic Kingdom!
As for me - never again would I do Disney on Independence Day! Good thing my Seasonal Pass wouldn't let me in that time!
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood