I go twice a year. If your going with Family, then there are specific places youd wanna see. If your going with significant other but no kids, thats another issue.
I like the big island for sites, volcanic scenery etc..Kohala coast is the best..
Maui is VERY chill, and has some nice sites. Check out Kaanapali there...
Kauai is the most original and non developed and tropical if you want something very laid back... Poipu beach FTW there...
Only place I WOULDNT go to is waikiki beach on oahu..basically Los Angeles with a nice beach. If you go Oahu, go north shore...
Whatever you do, respect the local culture. They are very sensitive to their heritage (rightfully so) and despise tourists who dont show respect and care for the islands heritage and history.