Great White Wall is the best, around Tavenuni. The soft corals there are snowy white along the side of a deep, clear drop-off about 250 ft.
The D/Ms use reef hooks at many of the other sites, so if you can, bring your own. At the white wall you will not need it however. At many of the other sites you would.
Bring a good light with you and a backup for the night diving. This is when the scorpion fishes come out at Fiji. If your batteries are rechargeable, you can get universal adapter packs at any travel-luggage store before you leave.
Currents are strong here due to the configuration of the islands. You should at least have a surface marker buoy with you, and you will probably use it at least once.
Right now is springtime in the South Pacific, so the waters will just be beginning to warm up. You will probably need a 3mm wetsuit, but no hood. Gloves are normally not allowed by the boat crews.
Sometimes hoods can be nice, however, for a little added warmth. Drysuit style hoods fit better than wetsuit hoods, the latter having extra material at the neck for stuffing into your wetsuit, which you really do not need for the warmer waters. I always bring a drysuit hood with me, and decide after the first couple of dives whether to wear it.
Overall, I believe Fiji has the clearest waters that I have ever seen, mostly due to the strong currents cleaning everything out nicely. This also feeds the soft corals and the many reef butterfly fishes and angel fishes. But the reefs tend to be smaller and so the dive boats tend to moor rather than drift dive. Therefore you need to work harder underwater to see a given location.
Bug repellant lotion for the return flight is something that you would never think of. However at the main airport at Suva, lots of mosquitos board the plane with everyone else on the tarmac. The terminal is not enclosed. You will want to put bug repellant on your ankles and legs before you board. The security will make you toss the bottle (it might contrain nitro glycerine after all, and your cell-phone could contain a detonator). So better to apply the lotion before you check in at the airport.