Depends on what you want. If you’re looking for an active nightlife or touristy things, then I’d pick Oahu or Maui. For pure beauty, and fewer people, Andrew’s pick is right on. Kauai has some vacant beaches, something hard to find on Oahu. There is a real nice cove on Anini Beach, in the north, near Hanalei. There’s usually nobody there and my wife and I really enjoyed it there (wink). Also, waterfalls are all over Kauai. The only navigatable river in Hawaii is there and it takes you to the Fern Grotto. And Waimea Canyon is one of the most beautiful sights you’ll ever see. That being said, I still really like Oahu. The things to do there are so diverse. But I’m happy wherever I am in the islands.
All the islands are beautiful. No matter which one you pick, you’re sure to love Hawaii.