What are the most challenging category climbs in cycling?

The most challenging category climbs in cycling are typically categorized as Hors Catgorie (HC) climbs, which are the highest level of difficulty. These climbs are extremely steep and long, requiring cyclists to have exceptional endurance and strength to conquer them. Examples of HC climbs include the Alpe d'Huez in the Tour de France and the Mortirolo in the Giro d'Italia.