There can be no simple answer. First, it is not clear that the hardest thing to know - if such a thing exists - has been found yet. Maybe it is too hard even to find. Also, different people find different things hard: not just in mathematics but in every aspect of life.
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AP CALCULAS AP CALCULUS* is not the hardest math. Analysis, Set theory, Algebra, Topology, Calculus and Number Theory
HL math
Pre-Calculus
It really depends on who you're asking....
Feynmans path integral formulation equations