I was born in Spain and I've been to Canada for a hockey tournament. Other than that, I really only know about USA.
As far as the "Greatest Country" I think that would depend on your context. Greatest in what aspect? I'm willing to believe that other countries may have us beat in some areas. For example, Japan might be technologically advanced. Australia might have better waves to surf. Mexico might have warmer beaches. Brazil might have better jiu jitsu teams.
Overall though, I think we're a great nation with a lot to be proud of. I also know there are a lot of ways we can improve.