Explore the top 10 easiest and hardest majors of 2017 to see if one interests you. only learn the subject they plan to teach (i.e. math or English) in its entirety.
Differential Equations is one of the hardest subjects imo. . today just to cover shit they should have known in an intro college algebra class.
Sort of subjective, but these are the top ten hardest math courses one can take on an undergraduate level from.