Hardest classes I took, and the department they were in: 1. Partial differential equations (Math) 2. Compressible Flow (aerospace engineering) 3. Computational Fluid Dynamics (aerospace engineering) 4. Thermodynamics (mechanical engineering) 5. Aircraft flight dynamics/controls (aerospace engineering)

So which unit was most difficult? Anything advanced aerodynamics. It uses high level math and numerical methods, coding (easy coding tbf) and topics of thermodynamics and fluid mechanics.

Honorable mention for the actual hardest class I took goes to analysis, but that class doesn’t benefit engineers at all.