What is the hardest math course you have ever taken? What made it hard?
Pre-Calculus, Calculus I are difficult, but the others are worse. Also, another very difficult course would be Probs and Stats (Calculus Based) because of the challenge problems.
I was told that abstract algebra aka modern algebra is insanely hard. According to most math majors at Lehman College, algebraic topology is the course that separates adults from kids.