We have done the July 3rd fireworks show twice. Best advice is get your spot early. It will be packed. Main street is your best bet. They have fireworks going over the castle and the train station at the same time.
I second this approach if you value having personal space and do not want to fight the crowds. You get all of the music and the all of the fireworks -- just without seeing the projections (which admittedly is a big deal).
Other than that -- the plaza gardens where they have the dessert party viewings are probably the best, but they come at a price.
The only time I've been able to get a good view was during a DVC evening magic thing back in January. It was pouring rain, so the park was relatively empty. I got absolutely soaked, but it was nice not having to fight the crowds.