GDP Vs Debt... How does one classify a country rich?

If anyone is asked with a question as which are the richest countries, the answer would normally be any developed country like US, UK, Japan, Netherlands etc. Even, I was thinking in the same way till, I hit upon some articles which showed the public and external debt content in some of the developed countries.

Interesting the top developed countries seems to have high content of public and external dept when compared to their GDP.

List of countries with high Public Debt

List of countries with high External Debt.

So as per wikipedia report, interestingly Libya and Algeria have low external debt whereas external debt are highest with US and UK.

If external debt is high, then how come the exchange rate of these country currency is strong when compared to other countries whose external debt is very low meaning well under control?.

Would be quite happy to receive an explanation from whoever is knowledgeable on world economy and currency exchange rates.