There is no definitive answer to this question as it depends on a variety of factors, including the skier's level of experience and ability. However, some of the most difficult mountains to ski can be found in North America, Europe, and Asia. Some of the most challenging ski runs are located in areas with high altitudes and/or steep slopes. Additionally, skiing in adverse weather conditions (e.g., high winds or heavy snowfall) can also make a mountain more difficult to ski.