The U.S. dollar has had a nice run. It's been the talk of global currency trading all year long.
And why not? After years of a weak greenback due to dovish Federal Reserve policy, the Dollar Index has extended big 2014 gains, up near-9 percent against a basket of major world currencies this year. Some individual currency trades, like USD/Brazil Real, have netted currency investors huge returns.
But to find what's arguably been the best global currency in 2015, an investor might need to put quote marks around the term "global currency."