Mine would have to be a tie between CS250 Computer Graphics II which had basically the hardest tests I've ever taken in it and which I only barely passed with a C- (you have to get a C- or better in order to get credit towards your degree for a core class at my college), and MAT258 Discrete Mathematics which I'm taking now and is probably the most confusing math class I've ever been in. Basically MAT258 is really high level math stuff which is normally used for doing mathematical proofs, and that is what most of the problems we get are, stuff like:
Prove that between every two rational numbers there is an irrational number.
CS250 covered 3D graphics and transforms and that stuff is just flat out hard, especially because our tests were just deriving stuff we covered in class (i.e. no multiple choice or true false, just proofs and deriving matrices).
So what is the hardest class you've encountered so far, and why did you find it so difficult?