What math class is hardest?

I'm curious, out of all the math classes you've taken or heard about, which one do you think is the toughest? Is it the one that requires extensive knowledge of calculus and differential equations, or perhaps the one that delves deep into abstract algebra and number theory? Or could it be something else entirely, like a statistics class that pushes the boundaries of probability and data analysis? I'm genuinely interested in hearing your thoughts on this matter.