Since I feel so strongly in my opinion, here it is...
As others have said, the best island depends on your preferences and expectations. However, know this... if you are dreaming of natural beauty, the most extravagantly rich in natural beauty is Kauai. It has the Na Pali Coast, boasting not the tallest sea cliffs, but definitely the most dramatic. It has Waimea Canyon, called the Grand Canyon of the Pacific, with endless hiking trails. It has the sunny beaches of Poipu. It has my favorite place on earth, Hanalei Bay.
Looking across Hanalei Bay, you see ocean waves, sea cliffs, and mountains laced with waterfalls after a rain. Stunning! Every time I head north from the airport just past Kilauea heading into the rain forest area of the Kalihiwai River Valley, a mixed feeling of delight and tranquility comes upon me. The feeling lasts all the while I visit Princeville, Hanalei and all the way to the end of the road at Ke'e Beach. No other island and, in fact, no other place on Kauai graces me with the peace that Kauai's North Shore offers.
On my last trip to Hawaii, this past January, I had to make my pilgrimage to this blessed North Shore before I did anything else. I was tired from my daylong flight and was not even thinking of what to expect. I just wanted to get to the resort at that point and relax. However, even in my fatigue peace came upon me again as I heard the waterfall just feet from the road when I passed it on my way down the Kalihiwai River Valley on my way to Princeville.
After doing first things first, enjoying Kauai's North Shore, I left Kauai to visit another island. My habit is to visit Kauai for at least two weeks and one of the other islands for one week annually. This year, the other island was Maui. To me, Maui is for those who love long beaches, lux resorts, fine restaurants, designer shopping, etc. Briefly, if you prefer wearing Tommy Bahama on your trip rather than jean shorts and a t-shirt, you are probably the type to prefer Maui or Oahu. If you would rather walk Main Street to shop than walk a wilderness trail, you are probably the type to prefer Maui or Oahu. If you prefer restaurants that require reservations, you are probably the type to prefer Maui or Oahu. Sure Maui and Oahu have their beauty, but beauty that rivals Kauai is only found in isolated pockets on the other islands. A day on Molokai or Lanai, for example, will probably satisfy almost anyone.
The Big Island is my second favorite. Oddly, not because it is what I would expect from Hawaii. Obviously, it is the only island with an active volcano. Treat enough. I usually spend a couple of days per visit roaming that area. It also has the odd landscape of recent lava flows. Very different and treat for a minute. Like the other islands, it has pockets of beauty rivaling Kauai's, which I have enjoyed thoroughly. However, I love it second to Kauai for none of those features. I treasure the rolling hills of Paniolo country the most. It is not the Hawaii that you expect, but beautiful nonetheless.
On this last trip, after my week on Maui, including a day trip to Molokai, I looked forward to returning to Kauai. I spent my first week at the Hanalei Bay Resort. Again, on my way there as I entered the Kalihiwai River Valley, I enjoyed my feeling of transcendence once again. My final week on Kauai was at The Point Resort in Poipu. The highlights of that week included hiking the trails of Waimea Canyon and Kokee State Park, an experience surpassing any of those offered on the other islands.
Another highlight of my final week was the one I enjoyed the last full day in Hawaii. On that day, I left Poipu and for the final time this year, I journeyed to Kauai's North Shore. This last visit I cried as I passed through the Kalihiwai River Valley. Not just because it would be my last visit this year, but also because it was then that I found how renewable the feeling the North Shore brought was. On no other trip had I passed that way for the "first time" so many times. After a week away, coming back each time was a renewal. To be struck by the power of this feeling by its ability to be renewed so often offered an even greater appreciation for this beloved place.
After stopping for an early lunch at the Polynesian Cafe in Hanalei, I traveled on to the end of the road. After a few stops on my way renewing my memory of the wondrous shore, I spent most of the day at Ke'e Beach, my favorite beach in all of the islands. As the day waned, I started back. However, before I left the North Shore, I made my way down the path to Pu'u Poa Beach to admire the bay with the mountains and waterfalls beyond as the sun set.