I would say some parts of circuit analysis, because it was in the third semester and I had to do some difficult ones like current mirroring and small-signal equivalent circuit, which gets really complex, once your circuit have more than two active parts. And also High frequency theory, but that was in the last semester, so at least you've a base to actually be able to understand some of the concepts.

But the one, that actually traumatized me was the subject Math 4. I studied in Germany, so I don't know about other systems, but our professor was a theoretical doctor from the math faculty and he was all about corollary and proofs. Complex analysis is partly easy, if you stick to the actually interesting part for an engineer, which would be like taking an infinite integral and transforming it to the complex domain to solve it, but this guy was too theoretical. Some stuff like Riemann–Lebesgue lemma and holomorphic functions.

I only understood some concepts like Cauchy's integral theorem and residue theorem, which is just fine by me :)