If you haven't been to Hawaii before - I would probably choose Oahu: Lots of touristy stuff (Polynesian Cultural Center, Pearl Harbor, snorkeling, beaches, etc, etc.) that everyone should see at least once.

But if you have already been to Oahu - I would instead choose one of the other two.

Maui is a great place for activity-oriented stuff like snorkeling, beach time, visit the top of the volcano (Haleakala) and ride bikes down at sunrise, good food, etc.

Kauai - Never been there, but is where we want to go next. Heard it's a great place. I hear it's nice for hiking and a romantic, beautiful place to stay.