Went to Maui for the first time in April and thought it was paradise....We will try Kauia next year to compare.
My first visit to Hawaii (Maui) was pretty memorable. We had a 10 day inter-island pass on Hawaiian and so went to the big island, Hawaii, twice Kauai twice and to Honolulu for a short visit and flight back. Only time I've ever had two cars rented on the same day!
I believe Kauai has even more natural beauty than Maui, but its very quiet and secluded. I remarked in another thread that everywhere has issues and even in Maui I'd probably get tired of the perfect weather day after day.
I've also very much enjoyed Tahiti as well as a Greek Isles cruise with visits to Pompeii, Rhodes and other ancient attractions. I think I enjoy both natural places and historic places with great monuments. Examples are the great National Parks of Utah for natural beauty and Venice for monuments.
Its hard to find a best, but Maui is certainly up there. Going for my third Maui visit for two weeks in November.