I’ve dove off Oahu, Maui, Kauai, and Hawaii. If I were to return, I’d probably go back to the Big Island of Hawaii for several reasons. I enjoyed the diving, especially the night manta dive. I went with Jack’s Diving Locker and really liked them. There’s a great variety of landbased things to do around the entire island.
That said, the only island I have no desire to return to is Oahu. Too many people, and the diving wasn’t that spectacular. If you go to Kauai and it’s have the right weather, check into going over to dive Nihei. It’s as close to virgin diving as you can get in the Hawaiian Islands.