If you're looking for diving only, Kona side of the Big Island. Good diving just about anywhere you can get in the water if you are a shore diver, lots of great diving without a long bumpy ride if you are diving by boat. Relatively little sand, even less soil, near the water in most areas, so viz is amost always good to great except on the worst of days.
Back in the early 80's (and later too, I worked in the pet trade and owned an aquarium shop back on the mainland for numerous years - just swim in the world's biggest aquarium these days) I was really into the marine aquarium fish hobby. In spring of '83 I came over and visited a fish wholesaler out somewhere in Pearl City on Oahu and talked the fish biz for an hour or so. He mentioned that most of the actual fish collecting was done in Kona 'cause that's where the fish are.
Anyway, each island has it's charm. Maui's pretty good, but the onshore diving I've done didn't have what I'd consider to be great viz or reefs. Lanai is a lot like Kona, maybe a bit light on the reefs on the dives I did, but great structure and deep water nearby, but it's a long bumpy ride. The crescent ends of Molokini are very nice, I especially like the sharks we saw when I've dove there and the back wall's cool, but I'm not personally keen on doing the same dives over and over and over. The little diving I did on Oahu was very unsatisfying... all out of Hawaii Kai by Koko head, I haven't done the west side stuff. Never have done Kauai, but I hear Ni'ihau is great (fairly advanced, not for newer divers) from friends who go over every couple years just to dive it.