My personal opinion, Maui. It also depends what you want. Although it was a long time ago, I grew up on Oahu and left just before middle school. My wife and I got married in Maui five years ago and spent the honeymoon on Oahu. We've been back to Maui each of the last 4 years and plan on buying a condo there this year.
Maui seems to be the island that has a little of each island and has the most amount of great beaches. Especially in south Maui from south Kihei to Makena, the best stretch of beaches anywhere. Starting with the three Kamaole Beaches, it's just one great beach after another. This time of year, the surf will be up more, which is great for body boarding, but may make it a little more cloudy for snorkeling. We always go this time of year, and some years the snorkeling has been ok, and others it has been great! It will be near the peak of whale season, so definitely book a tour with the Pacific Whale Foundation, which has tours leaving from Lahaina and Maalaea. (If you become a memeber then the tours are cheaper and you get a free one.)
To keep this short, I'll just say my vote is for Maui. If you would like any more info regarding Maui, send me a PM and I'd be glad to give you a full rundown on each area, places to stay, places to eat, things to do,how to save $$$, etc... My wife and I love Maui and enjoy sharing our experiences.