I add my vote for Conch. It's closest to the MK and has the best unobstructed views of the castle.
That said: if you're not thrilled with your view, a short stroll to the marina, or the boat dock, will also give you AMAZING views of the fireworks. It's usually a little more festive, too, because a small crowd (maybe a couple dozen people?) will gather to watch and ooh and ah.