Depends on how you define "hard." If "hard" means "I struggled in the class," I would say Structural Analysis II and Engineering Math II (partial differential equations). I fully blame the teachers, and not the subject matter, for saying that - because in subsequent classes where we covered that material again, it all made perfect sense. I just needed to see it explained differently.
As far as the most difficult subject matter itself, I would say my toughest class was Modern Physics. Some of that relativistic stuff was pretty abstract and hard for me to grasp.
As far as the most difficult subject matter itself, I would say my toughest class was Modern Physics. Some of that relativistic stuff was pretty abstract and hard for me to grasp.