European banks Deutsche Bank, Barclays and UBS have seen their market share in foreign exchange trading slump in the past year, as U.S. banks led by Citigroup (C.N) grabbed business, according to widely watched industry rankings. Citigroup kept its top spot as the leading foreign exchange trading bank with a market share of 16.1 percent, up from 16 percent a year ago, according to the Euromoney FX Survey 2015.