Kona side of the Big Island has got to rate pretty high. The entries for the best diving won't be sand entries, so there might be a little bit of work involved at some dive sites, but there are easy ones. If the Place of Refuge isn't the best overall shore dive in the state, I'd like to try diving any spot rated higher... step off the rock into 5 feet of water and you're on the reef. Currents aren't a problem for most of the commonly dove shore dive sites, and since there's relatively little sand or soil on much of the shore the viz is usually excellent from the get-go. Drop offs are very close to the shore, so it's easy to do a long multi-level dive.
The original question is a tough one because few people on this board have dove much on all the islands so there's really no comparing islands, I can say the one shore dive I did on another island couldn't compare to any of the shore dives (a couple dozen sites) I've done in Kona as far as viz, healthy reef, fish life and topography (but the access and entry were good). I'd like to get out more and try some other spots on the other islands for kicks.