USA, it is the most advance country in the world, infect this country is know for his culture, Society and its Freedom.
Peoples are going to work here because of his safe environment and of course its currency.
If you are looking today you are noticed that every country currency are continuously goes down but the USA currency dollar doesn't seem slow.
Its continuously growing faster and faster, which means if you are working here and earned money it will become more valuable then any of the currency across the world.