[Nil]

This doesn't have to be the most advanced, but whatever class gave you the most difficulty at any point in your life.

For me it would have been pre-calc. I ended up missing many days that year and along with being introduced to a different kind of math, it was the first time I actually ever failed a final. But of course, most likely out of sympathy for my situation, got a curve and passed with the minimum. Can't say I ever learned a thing in that class and was intimidated to take any other math courses for years after that. Fortunately, I've come to love math again.