It really depends on what you mean by best. Is it measured by the best player in the country, or four best players (like at the Olympics), or 100 best players, or amount of GMs or ... ?
And is it absolute or relative (e.g. in relation to population?). Is it historical or current?
Historically, the obvious choice is the Soviet Union. But how do you compare different eras?
Currently, India is a good bet. The fewer people we measure, the closer the US gets, until we measure just one person, then of course it's Norway.
Relatively (in relation to population), Iceland is often highlighted. I don't know if they are currently surpassed by others.
I know there are great players from various different countries, but if you had to pick one country as the "best," which one would it be?