Linear Algebra and Applications
If you have the right professor, this class can be incredibly interesting and expand your view as to what math is. Regardless, it is still one of the hardest math courses I have ever taken. Highly recommend, especially if you are considering a math major/minor.
Hardest class I took at Duke! The reason I am not a math major. DONT TAKE unless you love suffering.
This is a tough class - especially if you have never had a formal proof based course. Linear is the gateway course to all math classes and is graded traditionally (homework, 2-3 midterms, and a final). Get ready for many proofs - definitely do not recommend unless you want to major in math.