I’ve only been too Kauai and Oahu and I was surprised at how much I enjoyed the north shore. I think I’d go back there before I go back to Kauai.
Kauai is my favorite island, but I think even in a vacation I’d get bored after three or four days. I only lived on Oahu (not the North Shore though, because I’m not a millionaire) so it’s probably a slanted viewpoint, but my favorite of the major four to visit would be Kauai, the Big Island, Oahu, Maui.
In terms of where to LIVE, as a 35 year old man, Oahu, Big Island, Maui, Kauai.