Kauai. Oahu is very touristy and much more crowded. Lots to do and see but crowded. Kauai is beautiful and much more laid back but not as many things to do since it's tiny. Great if you love laying at the beach and snorkeling/diving. Maui is the best of both IMO. Plenty of things to do and see yet not as crowded as Oahu. Just my two cents.