The islands have different things to offer. Here's my very short version:
Topside: If you want to see an active volcano and have a pretty quiet vacation topside, go to the Big Island. If you're interested in beautiful beaches and a lot of different activities, with a bit more hustle and bustle, go to Maui. If you're interested in a lot of activities, a lot of hustle and bustle, and historic war memorials, go to Oahu. If you're interested in a totally laid back, not much do do, but gorgeous beaches, go to Kauai.
For diving: The Big Island has the night manta dive that's spectacular. Oahu has some great wrecks. Maui has Molokini and the ability to dive 4 islands. Kauai has lava formations and reefs. But any of the choices will likely show you the same marine life.
Once you've narrowed it down to the type of vacation you're interested in, we can help you with more specifics.