The AUD/CHF pair is the most difficult pair to trade because the spread can be pretty wide. This is due to the fact that the Australian dollar is a high-yielding currency, while the Swiss franc is a low-yielding currency. As a result, the spread between the two currencies can be quite wide.
Exotic pairs are the most difficult to trade because they have high volatility, which makes it very difficult to predict its future. Moreover, such pairs have high spread, making them expensive forex pairs to trade. As a newbie, it would be better to stick with major pairs, such as EUR/USD, USD/JPY, and GBP/USD.