From my experience of seeing hundreds of performances at Disney for fireworks.
Magic Kingdom:
*Directly in the center to the rear of the "hub". Any closer removes some views of lower level shots, but the shells are "seemingly" directly behind the castle in this view.
*The Bridge between the Hub and Tomorrowland. However, you get a GREAT view but the shells seem to come up to the "right" side of the castle.
*The Train Station or just below the station on the ground. This is a great spot, but know that the lights on Main St. are not turned off during the show, which could cause a problem in view.
*Ferry Landing. You may not know this but the Ferry landing has music close enough to where you can hear the audio music of the show. The view from here is spectacular and rivals the best view in the park.
Epcot:
*Just as you come into World Showcase, to the left and to the right you'll see stores. Prime spots are up against the railing just to the "far" sides of each store as you enter World Showcase. This is close enough to where you can actually feel the heat. Do not wait on the area just in front of the world showcase entrance as it is often closed off during the show and you may be bumped back further.
*Judge the wind before the show, if you are in the smoke like you will miss the entire show from coughing.
*Do not get behind one of the "islands" as it will block your view.
*The area sticking out towards the water in Italy is a great spot.
*Do not go in any area where trees are directly above you, especially if video taping, as during high shots it will get in your way.