The best place outside the park to watch is the California Grill, or the observation desk right off of it. The observation deck at the Contemporary would be, in my opinion, second best.
A kind of little secret place to watch is at the Grand Floridian. There's a strip of grass between Boca Chica and the lagoon, where you can spread out. It's not a beach or anything, and there are no chairs there, but you're much closer to the MK than the Poly. Also, from what I can tell, many of the fireworks are actually shot not directly behind the castle, but sort of to the west of the castle, and so you're directly in line.