Define "Best"? The more powerful military, probably. The best economy, perhaps. The best country in the world to live in? It is very subjective, and many indicators (violence, poverty, inequality,...) can be counter argument.
This is only really true for the United States as an entity, the story is different when looking at quality of life per citizen. When looking at all developed countries, the United States does not treat the poor well, with private prison systems and expensive healthcare and such.