I have been to Hawaii many times. So far I have visited the islands of Hawaii (Big Island), Maui, Oahu, and Kauai. All of them have their unique characteristics, like beaches, awesome snorkeling spots and volcanoes. I think my favorite island is a toss up between Maui and the big island. The beaches of Maui are really nice, plenty of white sand and good surf. The big island is very rocky, although there are some nice beaches, like Maniniowali and Hapuna. The rocks provide for very good snorkeling, since there is less sand to be kicked up by the surf.
I have only been to Oahu one time. I do not think I would go back, unless it was free. Although Waikiki was nice, there was too much traffic, and too many people. It is great for tourists who like the touristy thing though. The traffic on the interstate was horrible. It reminded me a lot like southern California. Kauai was nice, although it was small. Nawiliwili bay was very nice. The surf was medium sized, which was good for body surfing, but it was mostly shore break and therefore hard to consistently catch a good ride.
I have been to Maui a few times. It was the first of the Hawaiian islands that I visited. Haleakala, the extinct volcano is very nice. I had the privilege of riding down the winding road on a mountain bike, and it was one of the most exhilarating things I've done in Hawaii. Flying down the mountain at like 30+ mph is great. The big island is probably the most interesting island. It has volcanoes, beaches, rocky shoreline, and plenty of other fun things to see and do. I like it there, and will go back for sure!