depends on what kind of beach you like most..surfing beach? snorkeling from beach?
lying on beach with lots of people...or all alone?...here are a few on oahu, waikiki is famous for the diamond head view but is very crowded only good if you want to learn to surf or love to people watch. Hanauma bay is the only beach that charges but worth it if you ever wanted to snorkel, you have to sit through a short eco-movie before they let you in but its worth it, beach can get crowded because of tours going there. My favorite beach is Lanakai 30 minutes from waikiki but a world away from tourist stuff. Awesome views, not crowded at all, popular with kayakers because of two offshore islands close enough to paddle around...also popular with lesbians (at least i think they were lesbians, i didnt want to ask and get beaten up)(these female paddlers had muscles!!)... only downside is not very commercial (no beach bars or resturants)
maui has many beaches...once again it depends on what you like most in a beach....for snorkeling, ulua near wailai or black rock at kaanapali.
for windsurfing there is a worldclass beach on the north shore, i forget the name but it is near paia, for picturesque try the kam beaches in southwest maui near kehei there are 3, for local flavor try makena which is probably the biggest beach on the island , north makena is topless
for kaui try poipu great for kids but has everything an adult could want in a beach
havent been to hilo so cant help there
oh and for a great tour if you into snorkeling go to molokini crater offshore from maui it is one of only 3 in the world like it.