What is the most difficult topic in HL Maths?

It will mainly depend on your own interests, so there is no definite answer. In my opinion, the whole calculus section is probably the most significant part of the course as it will take up a significant part of lesson time. In the exams, questions related to calculus will come up quite frequently, especially in the more challenging questions. If you do HL, the option chapter can be quite challenging as well. Nevertheless, especially in Maths, I believe that practice does make perfect.