I visited Nan Madol in 2007 and ever since I had thought it was not only worthy of world heritage status but probably the most deserving site I had been to which wasn't listed.
Getting to Micronesia isn't easy, and getting to Nan Madol takes a bit of effort once you are on Pohnpei, but it is well worth the effort. Expect to be alone when you visit or be among a very small number of tourists.
It has been called the "Venice of the Pacific" for good reason. The small islands and structures separated by canals evokes images of Venice.
I think that Nan Madol is the top attraction in the Pacific, perhaps even surpassing Easter Island.