My two cents...
Kauai is by far the best for outdoor enthusiasts. It has the best hiking. No other island comes close... Kauai is beyond a hiker's wildest dream. Kauai offers the most beautiful beaches as opposed to the most active where Oahu excels or the most expansive where, perhaps, Maui excels. A traveler couldn't possibly fit all of Kauai's paid adventure activities, where it reigns supreme, into one week, and would find it challenging to fit them all into a two or even a three week vacation. Kauai excels in such things as four wheeling, zip lining, air tours, boat tours, water skiing on the river, tubing down old irrigation canals, kayaking, dreamy jungle pools for swimming, horseback riding, etc., etc., etc. Although Kauai offers good fishing and snorkeling, Maui offers the best of both. Unless one is a professional-grade surfer, Kauai features great surfing. Kauai offers all that the average and certainly all that the beginning surfer can handle. For golfers, Kauai offers several acclaimed courses.
Moreover, Kauai has more natural beauty than all the other islands combined... for those who may disagree that Kauai's beauty is that much greater than the beauty of the other islands, I'll admit that I may have slightly exaggerated.
I am always baffled when people say that there are more things to do on Maui and Oahu. I can only imagine they are limiting their ideas to shopping, fine dining, night life, and, perhaps, small scale theme park type amusements. I don't go to Hawaii for any of that. There are places that offer those types of activities in far better quality than Hawaii. If you want the best shopping, consider New York City. If you want the best night life, consider Vegas or Miami. If you want the best amusements, consider Orlando. If you want the best fine dining, consider Paris, New York, Vegas, or San Francisco.
Vegas has world class restaurants, night life, shopping, and amusements. That is probably why Vegas is the number one destination for Hawaiian residents. When they want that stuff, they leave paradise and go somewhere else. Hawaii is ideally for nature lovers which is most concentrated on Kauai where the best in nature and natural adventure excursions are offered in overwhelming abundance.
Yes, I know, some people (perhaps, most people) want a dash of all the above. That's why Maui is so popular and that is why Oahu undid itself trying to accommodate that taste. Perhaps, I should stop glorying in Kauai's superior beauty and adventure as my contribution to its preservation.
