I have never been to Rabaul, PNG, Munda & Solomon Islands, but I was in Palau in December 2017 with Palau Aggressor II. I enjoyed the trip as much as Maldives. Each has unique sites that you don’t want to miss. In Palau I like German Channel, Peleliu Cut & Blue Corner. In Maldives I like Fish Factory, Maya Lagoon, Rasdhoo Atoll. Both trips were with Aggressor liveaboards which were very well organized, great food & excellent customer services. Top side Palau with mushroom like limestone islands is more scenic than typical tropical lowland Atoll island in Maldives.
Raja Ampat is better in terms of bio diversity & healthy & pristine coral formation. I noticed some coral bleaching & destruction in Palau & Maldives. Topside of Raja Ampat, especially in Misool area is very similar to Palau.
I’ve been to Raja Ampat 7 times. So, you can tell how much I like Raja Ampat. The eight time will be in January 2020 with Amira. I think there is still 2 spots available for that trip. Send me a PM if you’re interested in joining me then.