I've seen the fireworks from all the different spots and each has its own charm. The hub, hands down puts you right in the mix...projections, firework spinners on the castle, cannon shots from the matterhorn, green haunted mansion room effects, and the laser effects for the space mountain sequence...
Rivers of America has a nice view of the fireworks, accessible view of the matterhorn for the pirate cannon shots, use of lasers from tom sawyers island, but no projections like those on the castle. Personally, I think the best sequence has to be Frontierland, when fireworks go off behind Big Thunder Mountain and the train noise effects go off. This area is relatively crowded due to it being wedged in between the two Fantasmic showings and people coming and going...
IASW has a good view of the main fireworks. You do get projections on the IASW facade, which is pretty neat, but no good surrounding effects such as the haunted mansion fireworks, the cannon fire from the matterhorn, or even the laser effects for space mountain. The beauty of this location is ample sitting/standing space and very little crowding.
The elevated train station has a great view of the main fireworks. You really get the full bloom effect of the fireworks from this position. Unfortunately, extra effects are really tough to see from here. Projections, matterhorn cannon fire, haunted mansion, space mountain lasers, etc. I personally like this place best for low crowds.
Sorry for the thesis, just got into a typing spree heh.