Have done the GBR, Fiji, and Kona (x2). There trip reports, galleries and slide shows for each at our website.
Issues: 1. Depends where you go on the GBR, we went to the Ribbon Reefs, so it was incredible and no other divers. But you have to do a live-aboard. 2. Not been to Maui but Kona diving is supposed to be better than Maui. In addition almost all land based. 3. Fiji again experience can depend on diving from a resort or live-aboard.
But to make sweeping generalizations....
The GBR is the best of the three as it just has more of everything! More sharks. schools, grouper, etc. But fewer mantas. Very good macro. Fiji we did a shark dive at the end of the trip but did not see many in our typical diving, did have some passes of manta but none showed at the cleaning station. Really big draw for Fiji is the Corals, especially the soft corals.
Kona is more about the topography, small fish that are unique to Hawaii and must note has only hard corals, no soft. Very few sharks BUT see a manta now and then and of course the famous Manta Ray Night Dive which is still in my top ten dives (out of over 4000).
#1 GBR
#2 Fiji
Must say a Distant
#3 Hawaii