Hello all, haven't posted in a while but wanted to chime in having been to PNG, Indo and the Solomons.
PNG is my favorite, though I haven't been in quite awhile, so it's good to hear people are still having good experiences. PNG has it all, high diversity, big stuff, WW2 wrecks and amazing scenery with lowest population density of all. I've been from Eastern Fields to Kavieng and everywhere in between, best diving I've ever done. Airfare is outrageous by the time you figure in domestic flights. It was always bad, but is worse now, no excuse considering the price of oil the last 2 years.
Komodo, Alor and Ambon Indonesia are a close second, but not as much big stuff. Diversity is off the charts and plenty of fish, sharks and tuna if your operator has good local knowledge.
Most recent trip in September of last year was to the Solomons, it certainly has plenty of WW2 wrecks and lots of fish the further you get away from Honiara. Liveaboard is probably the best way to see it. I was on Taka, very well run boat, had sharks on most dives, some very big tuna and bumpheads. I heard many good things about Uepi, but not sure how long the boat rides are or how nice their boats are. Taka has itineraries that include Uepi dive sites. So one could go either way. Definitely not as much macro species diversity, but plenty of biomass and there's always some odd endemic stuff to see, just not to the degree of what I saw in the shallows at Restorf island in Kimbe PNG or the hard coral gardens in the waters north of Alor.
The 3 leading indicators of the best diving IMHO: proximity to deep water, high species diversity and low population density..