I'm partial to Maui, mostly because you can dive 4 different islands from there (Maui, Molokai, Lanai, and Molokini). There are also gorgeous beaches and lots of things to do topside when you're not diving, plus there's good shore diving as well.
I agree with this 100%, but for quality of diving alone with no consideration for the rest of the island, the Kona area holds a slight edge for viz, fish, and topography IMHO. friscuba will show you a great time on his boat if you choose the big island.