Hands down, Trigonometry. I have no idea what it is or how to use it even after spending an entire year in a class that was teaching it. I was bored so I channeled my creative energy into being a disruptive influence. The teacher, Father Carson, slugged me upside the head from my blindside while I was chatting up a classmate. I scored a 26 out of 100 on the final exam. Before he turned in the final marks, he asked me what my plans were, if and when I ever graduated. I said I was going directly into the Air Force and hoped to be a Pararescueman. (I ended up in Security Force). He looked me up and down and said "God save us all." He gave me a 70. He later told me he never flunked seniors. I did just fine, 80 or above, in every other class. I also hated Geometry. The grand irony of that is that I would spend 30 years of my life solving geometry problems as a Planner & Estimator on a daily basis.