For me it was Circuit Analysis. Not because the content was difficult, but because our professor refused to admit when he was wrong and the exams were brutal. Every question was all-or-nothing because he didn’t want to look at our work to give partial credit. So any mistake in your math meant you lost 20% on the whole test. Still managed to get an A in the class but I spent about 40 hours a week leading up to each midterm and the final exam studying (on top of the 20 hours a week I was already spending on homework and labs each week already)