Palau is awesome. I did the Aggressor liveaboard.

The island itself is very beautiful, and the natives are extremely friendly. If you can't swing the liveaboard cost, would recommend staying on the island and going out on daily boat rides. And don't miss a chance to take a couple of land tours of the island. :)

Truk/Chuuk is awesome, but not an island you'd want to stay on for very long. The natives aren't very friendly. You'll have to stay in a hotel when you get off the boat, in order to have some time between the boat and the next time you fly. I recommend spending as much of your time on Truk on a liveaboard.

Aggressor might still be doing dives at Truk. I know things got a little dicey after 2003, so... When I went, in August 2003, they had a guard who watched the boat at night, rifle in hand. We did not stay at port overnight... we stayed out in the lagoon.

It's been almost 4 years since I was there... things might have improved. Talk to the liveaboards who service Truk.

The wrecks and the shark dive(s) are just incredible at Truk. If you can safely go and see them, do it.

In no particular order

Sea of Cortez -close by and inexpensive. Cousteau called it the worlds aquarium
Galapagos - see it before it's gone (I still haven't)
Antartica (under the ice) - never done it. Only seen pictures and it looks amazing, this would be a real dream dive for me.
Bonaire - Fabulous, easy shore diving.
Hawaii - the Big Island, amazing diversity, don't forget to dive the Hilo side as well as the Kona side