If i were to go to Hawaii just for the best camping. Whats the best island?
No part of Hawaii is Amazing. Big Island may give the best options but most places aren’t legal, even though a lot of locals use them.
Maui is awesome if you want to go into Haleakala
I’d recommend Kauai and Big Island for the best camping! 
Do your research, if you have to get permits, get them! If you show up to a place and are not welcomed or there are no trespassing signs, please leave, always respect the land the people.