If you'd like to view the fireworks from outside the park (which means you'll be missing the projections on the castle), my favorite location is the ferry dock at the TTC. DH and I discovered that location by accident, one night when we left MK early to get ahead of the crowds! Disney pipes in the music, and there's a great view of the castle (too far away to see projections, I'm afraid) and the fireworks around it.
Other locations for watching MK fireworks are the beach at the Poly, the marina and boat dock at GF, and Geyser Point at WL. The music plays there, too, and snacks and drinks are nearby as well.
Edited to add: For Harmonious at Epcot, you can see the highest fireworks from the bridge between BC and BW, near the IG, and also from Topolino's deck at Riviera (but I think you need to have an ADR at Topolino's to go out there).