The strongest traded currency of 2013 was the euro, according to CNBC. The euro was up 26 percent compared to the Japanese yen of that year. The exact measurement of a currency's strength depends on the comparison between individual currencies.
The euro saw more than a 4 percent increase against the U.S. dollar, despite the U.S. dollar enjoying its best annual gain since 1979 against the Japanese yen. This action allowed the euro to have the most gain during 2013 against competitive currencies. This action surprised hedge fund companies, which had expected a weak euro market in 2013. As of 2015, 19 European countries use the euro.