It is entirely subjective. My hardest was maths which, for me, felt like a continual struggle for two years. Every time I thought I was improving I would be introduced to a new topic I didn't understand. However, I know there are a lot of people who really understand maths and have no trouble getting great results.
Apart from maths, my other three subjects were essay-based and I didn't struggle nearly as much with them. So from my point of view, maths or a science-based subject would be the hardest, but on the other hand, someone who really excels in the sciences might have a lot of difficulty with an essay-based subject. It depends on your individual strengths.