I don't know if you have a specific island in mind (or touring several), but we love Maui and have heard many others (who've been to all of them) say it's the best one. Went there for our honeymoon 13 years ago, and have been back 4 other times. Lots to see and do, lots of beautiful beaches and great snorkel spots.
Have also heard Kaui is beautiful and more quiet. Honolulu (Oahu?) is busier, more like being in a city rather than a small town. Molokai is rainy, but lovely; not sure if there is touristy stuff to do/stay there? Just depends on what kind of experience you'd like.