Well, we can't really say which skill is the hardest for all students. Everyone finds different aspects of maths challenging, so what might be easy for you could be hard for someone else and vice-versa.
That said, Skill 851 - Non-calculator trigonometry (7), has more building blocks than any other skill on HegartyMaths.
If you set skill 851 as a challenge for yourself, you would need to complete 189 building blocks before being ready to tackle this skill! That's 20% of the whole site!