Depends what you want to see...
I've found that Showcase Plaza is the best of all the places I've watched it from to see the images on the globe in the middle, but it's also usually pretty crowded.
The boat dock in Morocco was good for seeing the fireworks. Not too crowded at all, but a little obstuction from the trees in the lagoon.
Probably some of the best locations are the areas on the water in the UK, Italy and between France and International Gateway. Most of the time I seem to be there thought, they're being used for private functions or wedding receptions.