Love Maui. Our favorite of all the Hawaiian islands. Lots to do, great beaches, great restaurants, great shopping opportunities. Nice people. Not too crowded, either. Oh, and the whales - tons and tons of whales. Fantastic.
Kauai is by far the prettiest of the Hawaiian islands. That doesn't mean it is without issues, though. Lots of chickens all over the place. Waimea Canyon, Barking Sands beach, Hanalei, Poipu beach - all are worth visiting. it's a very, very beautiful island.
They don't call it the Big Island for nothing. The Big Island is bigger than all the other Hawaiian islands put together. It's deceivingly large. The volcano is a must-see. If you can get to the Kona side, do so. Lots of great black-sand beaches and some of the most fantastic golf courses on the planet.