Please refer Below Analysis and decide the Hardness Level of Each Subjects according to the performances of Students.
If Pass Percentage is Low, it should be hardest.
BUT
if you need to think about the Z Score Ranking, Compare the A,B,C Results in Each Subject. When Minimum Students have taken A,B,C Results in a Subject, then the higher result of that subject will increase your z score.
Click Below for Subject Wise A,B,C,S,F, Results