What country allows for the least restrictive world-wide travel and ease of acquiring dual citizenship?

Hi- I'm a natural U.S. citizen, but I'm contemplating getting dual citizenship.
I'd like to get my second citizenship in a country which (ideally) would allow me to travel without restriction to any other nation in the world, and for which citizenship is fairly easy to obtain (and without a great deal of cost).

As an example of what I'm trying to avoid: I recently tried to enter mainland China from Hong Kong, but was denied entry while all of the Aussies were waved right through.

...Since I'm not a terrorist, and could care less about the politics between nations- I want to just be able to go wherever I want, whenever I want, plus be able to enjoy all of the other benefits of dual nationality without having to also give up my U.S. citizenship.

Also, any references you have on the matter would be appreciated.

Any citizenship will request some kind of contribution from you. Either money, tax or time.

But supposely New Zealand passport is the easiest to be used around.