Well, I am sure if you do a search you will find tons of information and opinions. Of the usual islands (not including Molokai and Lanai since most people don't stay there) the Islands rank in order as follows: Big Island, Maui, Kauai, Oahu. As far as operators, I have posted some information (and links) to various operators on the Big Island and Maui. You can find this information in the Resources for Divers link on the left hand side of my opening page. If you are booking for more than a couple of days, many operators offer discounts.
As far as trip planners, that is really variable. I have gone through SunTrips, Pleasant Hawaiian and Expedia. They were all good. It just depends on who has the best deal at the time.