Most of the time you can watch from the Poly beach. The exceptions are the very busy times such as for 4th of July, New Year’s Eve, etc.
Other spots include the GF marina, the GF boat dock, and the 4th floor observation deck at CR.
But my favorite place is the ferry dock at the TTC. The music is piped in (as it is for locations above), and you have a pretty much straight-on view of the fireworks centered on the castle. We discovered it one night when we were trying to beat the crowds out of MK. It worked, too - we watched the fireworks and caught the first parking tram to our car, before the hordes began exiting the monorail and ferry.
Speaking of parking, sometimes the monorail resorts won’t let you park unless you have an ADR or some other reservation there. So either book dinner at Poly (‘Ohana or Kona Cafe) or whichever resort you’d like to watch from, or park at TTC and ride the monorail to your selected resort.