“Everyone finds their own Island” is a Hawaiian saying I love. Everyone's Hawaiian vacation desires and expectations are different and so 'one size fits all' advice doesn't really apply (as you can see from the responses). We've been to Hawaii a bazillion times (my wife went to UH Manoa and we still have friends and family there).
I will say that O'ahu is a great choice for 'first timers' to the islands...particularly now that MVC now has both a location in Waikiki and the more remote Ko Olina. Splitting your time in Hawaii between 3-4 nights in Waikiki and the rest at Ko Olina (or on another island) would make for a great vacation.
Honolulu is a multicultural urban paradise with a plethora of Hawaiiana and WWII history and some of the best food in the world. The rest of the island is home to beautiful mountains, beaches, and towns all more or less linked back to the mother city. From the Waikiki MVC it is easy to visit most of O'ahu's major attractions - Pearl Harbor, Iolani Palace, Diamond Head, the Bishop, Hanauma bay, Queen Emma Summer Palace, the Pali lookout, Punchbowl, etc., etc. Lots of stuff to see and do that is walkable and an even wider variety of delights a short trolley, bus, or taxi/Uber/Lyft ride away.
Kauai is the lush tropical paradise of most people's dreams of Hawaii and I can't recommend it highly enough. Lots of opportunities for walking/hiking and water activities off the North and South coasts and on Hawaii's only navigable river. Waiohai is our favorite MVC Hawaiian resort and sits on Poipu beach - one of the world's best beaches with a reef, snorkeling, swimming, wildlife, etc. Kauai Beach Club sits on a bay not the ocean and has calmer water and an enormous pool complex. Lagoons has beautiful 2BR and 3BR villas but no beach or onsite dining.
The Big Island is beautiful....but for better or worse the MVC is located in the planned resort community of Waikoloa. The beach the MVC resort sits on (A-bay) is fine but there is a wonderful beach about 15 minutes north (Hapuna) and the towns of Hawi, Waimea, and Kailua-Kona are close-ish and worth visiting for dining and shopping. Farther afield are the snow, volcanos, pasture, orchids, lava, coffee, waterfalls, snorkeling, and taro fields that make the Big Island unique. But all those are quite a drive - the Big Island is *Big*.
Maui Ocean Club has great pools and sits next to a beach which isn't great for swimming but has good snorkeling up by Black Rock. It is in Kaanapali which has unfortunately devolved into a dense suburban sprawl. But, once you get out of that area Maui has some of the most amazing scenery and outdoor activities in Hawaii - Hana is enchanting, Haleakala revelatory, and Molokini is our favorite snorkeling spot in Hawaii.
There are also five Sheraton and Westin Hawaiian resorts now available through Abound Points at which we have not yet stayed. The Sheraton Kauai Resort is reasonably nice and across the street from the beach, Princeville has lovely villas but no beach, and the three in Kaanapali are not appreciably different than MOC (unless you are staying in a studio in which case the Vistana properties are superior).