For MK fireworks, what you would like, if at all possible, to be as close to the extended line that runs from Cinderella Castle through the center of the Train Station. To an extent this will be on the Poly beach.
At GF there is a pier near Narcoossee's where you can go; however it is nowhere near the center line and the view is not as good as from the Poly Beach.
Also, if you can get into 'Ohana you can get a good view and you get the music and narration piped in but do not get the noise from the fireworks.