If I had to choose one island I would pick Kauai. No, maybe the Big Island.
Kauai has great beaches. Is less crowded. Has the Napali Coast. Best of all, it has the best Zip Line I have tried.
http://www.adventureskauai.com/ The bad thing about Kauai is that food is a little more expensive because it has to be shipped in from the other islands. However, Costco does still have the $1.50 hot dog/pop deal!
The Big Island is the most diverse in that it has I think 10 or 11 different climate zones. We stayed in Kona for a week then we drove over to the Hilo side for a night and stayed at Volcano House, which is right next to the crater. It was the most amazing experience to wake up in the morning and look out the window. The visable lava flow stopped the day before we got there. It started again the day after we left.

The Big Island is also home to the most southern point of the US. We made the drive around the island and it was incredible. The black sand beaches, the rain forest, Mauna Kea. Everything was fabulous! Oh, they have a Costco as well with the $1.50 hot dog deal.
We have spent time on Maui and a lot of time on O'ahu as well. While Maui was nice, it wasn't our favorite. We always stop on O'ahu for a night and a day so we can see Pearl Harbor and the Missouri. My DH's cousin lives on O'ahu and a visit to see him would be the only thing that takes us back as it has become more touristy.
Which ever island you decide, make sure you pick up the travel book by Andrew Doughty for that island. They are not only dead-on with recommendations, but he has a great sense of humor. They are probably the best travel books I have ever purchased!
Which ever island you decide