I think it's like comparing a rain forest to the desert...Hawaii being the desert. There's a LOT of cool endemic life in Hawaii that you literally cannot see anywhere else in the world, but it typically isn't obvious and often requires a knowledgable guide to see. And of course, as with all diving, it may require some luck as well.
We've done well over 300 dives in and around Maui and still regularly get surprised by "firsts," seeing things we've never seen before. I think Hawaii diving is great, it's just different than other parts of the world, which is why I recommend strongly that a good guide is essential for folks unfamiliar with the marine life.
As far as what time of year, I would say go during whale season, ideally in February. If you aren't lucky enough to have an underwater encounter with a Humpback, the surface intervals provide great viewing.
As to which island, it depends on what you want to see/do. Oahu is known for its wrecks, Maui is known for being able to dive 4 islands (Molokini, Lanai, Molokai, and Maui) with Molokini crater having a 300' shear wall to dive, and the Big Island is known for the Manta Night dive. Maui and the Big Island have really good shore diving as well.
If you're interested in topside activities, that starts an entirely different conversation.