This may not sound good to some, but I would say China.
They have a large, growing middle class, have a strong military that has limited intervention abroad, exports more than they import, and is not a debtor nation.
However, they have a huge population with heavy concentration in the urban areas. A huge middle class consumer population could have a very, very difficult time being maintained as Earth resources are consumed and become more scarce. As countries that owe China default on their loans (it could happen), then they may have some military and economic challenges too.