I do not have a favorite island. Each island has unique characteristics that make it special. I like them all for different reasons.
I saw too much and did too much to go into details so I will just recommend a couple things.
Take a helicopter tour. We took our tour with Blue Hawaiian helicopters on the island of Kauai. They were very organized and professional. I felt safe and enjoyed every moment of it. The experience is worth the price.
Consider a submarine tour. We took our tour on Oahu with Atlantis submarines. They operate the largest passenger sub in the world. It is a unique and enjoyable experience. They also operate tours around the world. Check them out.