2 cents worth having been to Maui, Kaui, Oahu, Big Island: for scenery and more off the grid exploring opportunities I would choose the Big Island: more driving than other islands as the distances are further but as you are on this site I assume you like driving
The distances are also relative: you could drive around the whole island in one long day. You can be in 10 different climate zones inside of an hour.
Our Big Island trip was 14 days. First half we rented a top floor condo in the Puako area for 1/2 the trip (1/2 hr north of the airport, desert area, tons of beach options with NO people around).
http://www.hawaiioceanfront.com/condos.asp
Second half we rented a turn of the century house in a cool ex-sugar plantation town called Pahala, (
www.pahala-hawaii.com) down on the southern volcano side of the island. That way we could do tons of exploring, see the whole island, without spending the entire trip in the car.
Have fun, hard to go wrong!