hey all, for those who finished all their math reqs, what in your opinion was the toughest math course?

So far I've taken, calculus 1, 2, and 3, differential equations and I'm currently taking linear Algebra, and that is very easy. The worst course for me was calc 1, but that was probably because I really didn't put too much effort into honors precalc in highschool during my senior year.

Next week I'm allready signing up for classes for Spring and I'm not sure what to take...here are my options:

any suggestions?Code:Math 430--Formal Logic I (3 hrs) Math 435--Foundations of Number Theory (3 hrs) Math 436--Number Theory for Applications (3 hrs) MCS 421--Combinatorics (3 hrs) MCS 423--Graph Theory (3 hrs) MCS 471--Numerical Analysis (3 hrs)