The most difficult currency pairs to trade in forex trading business are typically the exotic currency pairs, such as USD/TRY, USD/ZAR, and USD/BRL. These pairs have lower liquidity and wider bid-ask spreads, which can make them more volatile and unpredictable. Traders may need to be more cautious and have a higher risk tolerance when trading exotic pairs. It's important to thoroughly research and understand the fundamentals of each currency pair before trading, regardless of their difficulty level.