Kauai is the best island for you based on the activities and qualities you mentioned. I’ve been to Oahu and Maui as well, but liked Kauai the very best for scenery, hiking, uncrowded and absolutely beautiful. It’s easy to drive to the various scenic spots. Some roads are narrow and one lane over bridges so don’t expect to drive fast and you really won’t enjoy the scenery if you do.

I traveled to Kauai, Hawaii and absolutely loved it. From what i saw and what i have heard it is the most natural beauty. Very scenic and more untouched. There were several waterfalls i hiked to and also some great views and beaches. I would definitely take a look into Kauai.