Just got back from Truk, Palau, and Peliliu. Was in Truk in 2006. If you want to dive the wrecks, do it soon. After almost 70 years they are really starting to deteriorate and collapse.
But the wrecks are awesome to see, along with the history experience. The Odyssey is the best experience, more dives per day, and on-board nitrox as well. Make sure you have a nitrox cert and understand your computer for deco, it's very easy to be in deco there as the wrecks are typically 100+, especially if you go looking around the holds and engine rooms.
Palau was nice, but it is a very long boat ride to the main sites, typically only 2 dives per day from a shore based op. More restaurants and things than Peliliu.
After staying on Palau, we went to Peliliu (1 hour boat ride) which is really close to the dive sites, typically a few minutes, so you can get more diving in, or at least don't spend all day for 2 dives. Also, the dive ops from Peliliu avoid the crowds, so it's a better experience. The WWII tour on Peliliu is really interesting as well. We stayed at Dolphin Resort, very nice setup.
As to PNG, haven't been there, but have been to Raja Ampat, the Indonesia side. The diving there is spectacular. Critters obviously don't see a lot of dive traffic, they are unperturbed by divers. Did that on the Paradise Dancer, 10 day liveaboard.
Have done similar diving around Komodo and Sulawesi. For awesome diving and very pristine sites, you can't beat a liveaboard in Indonesia.
So, I guess where you go depends on what you want to see. Truk is good for wrecks and if you are there, it's pretty easy to pop over to Palau via Guam. Note that it is hard to line up the various legs to go to Truk, then guam, then palau online. We went through a travel agent to get that all sorted out. Palau is great for sharks and turtles and some very nice walls and reef areas.
Anything Indonesia is huge biodiversity, tons of nudibranchs, lots of species of everything. And muck diving for some really bizarre critters.
Once you dive Indonesia, it's hard to find anything better. We are going next year to Wakatobi. Other sites are nice and interesting, but Indonesia is tops.
So try Truk/Palau this time around, then Indonesia next major dive trip. Best of the best!