Anything involving air law, at all levels (PPL(M), then JAR-PPL(SEP), and finally CPL. I passed them all first time, but near as dammit minimum mark on all of them, and hated all of them.
For me to learn stuff, I need to understand it. Air law is all about memory - it's why I was also useless at history at school, and foreign languages at most times.
G