To be taken with a large grain of salt.
The Economist (UK) have this big study; their article is here along with a table of the countries, in order.
It's summarized by the Washington Post. The top three countries, in order are Switzerland, Australia, and Norway. The USA is tied at #16 with Germany. Nigeria comes in last.
If you came into the world today and could pick your nationality, there are at least 15 better choices than to be born American, according to a study by the Economist Intelligence Unit. The firm looked at 80 countries, scoring them across 11 variables to determine “which country will provide the best opportunities for a healthy, safe and prosperous life in the years ahead.” The results, mapped out above, are both surprising and not.
Both articles are really quite interesting.