We’ve been to the Big Island the most, and it’s our favorite because of the diversity of scenery and things to do. Kauai is my next favorite because it’s gorgeous - so lush! Maui’s nice too, especially if you can do a sunrise bike ride down Haleakala and make the drive to Hana and beyond (especially if you’re like me and had to see Charles Lindbergh’s grave). When we first started traveling to Hawaii in the early 90s, Pearl Harbor was what we did on going-home day between flights from the BI and to home, but now I think we could easily spend two days there, and besides Pearl there’s the Polynesian Cultural Center (if you’re interested in learning about Polynesian culture and history), the Bishop Museum, and the giant waves on the North Shore (we watched the surfers, didn’t join them).
So as others have said, there’s so much to do in Hawaii and the islands are so different from each other that it’s hard to choose just one! But since time is limited, I don’t think it’s wrong to skip Oahu. Kauai and the BI are great choices. Maybe you could fit in a visit to Pearl Harbor, but if not, there’s your reason to return.
Re Aulani: it’s drop-dead gorgeous, and if I ever manage to stay there on points I’d love to do it. Disney (Joe Rohde) did an amazing job making it look Hawaiian - the other resorts near it and on the other islands could be in Miami or in Southern California or any other beach resort in the world. But it’s exceedingly expensive, and for cash isn’t worth it. We just made reservations to stay for the second time at the Marriott down the road, for half the price. We can visit Aulani and eat there if we want, and it’s the same ocean for swimming!