It's hard to rank the courses in matters of importance because they are all important and have different applications. However, I would lean towards optimization as an essential and exciting course.
Reason: It's the mathematics of finding an optimal (best) solution from all feasible solutions. Optimization application ranges from mechanics, economics and finance, operation research, machine learning, etc. I find its application invaluable and relevant, especially for an aspiring quant.
Fact: I took an operation research class as an undergraduate and loved it.