My vote would be Kauai, and specifically between June 26 - July 9, 2010, because I need a divebuddy :-)
But to be fair I’ve heard great things about most of the Islands. I dove in Maui last year and enjoyed both Molokini Crater as well as the Island of Lanai. While I was there on a charter I was partnered with a tourist who had just gotten back from a week in Kona (big Island) and showed me pictures of his Manta Ray night diving experience (amazing - a must do on my list of future dives). But for this summer I’ve settled on the garden Island of Kauai, as much for the land activities as the diving. I’ve heard from many people that the best diving in all of the Hawaiian Islands is reached from Kauai to the "forbidden" Island of Ni-ihau. I’ve actually signed up already for the Ni-ihau trip with Seasportdivers for Tuesday, July 6th. Also planning several charters to dive sites like Sheraton Caverns, plus some shore dives.
Let me know if you decide to do. I don’t think there is a bad choice.
Rich