I like the remoteness of Kauai and Waimea Canyon best. The wild chickens are a story in itself.
But, on the Big Island snow on Mauna Kea and the active volcano are very interesting.
Maui is great for the beaches, restaurants, and Mount Haleakala (feels like you are on the moon). The road to Hana is a must do, but the "no rental car" road is even more exciting (maybe because you are not supposed to be there).
Oahu, except for Pearl Harbor, is my least favorite. Too much like a big city and crowded.