POF was the hardest in my opinion, was questions and answers that could be very tricky to understand, half of it because of the way the sentences were formulated. English not being my first language it could be tricky.
Gen.Nav and Met seemed hard when started, and to my teachers and my own shock I get 100% on both of them!
Looking back at all the exams, probably could have done better on them all, but getting 1 to 3 wrong per exam was not that bad either.
Airframes and Systems was also pretty hard, because of the 4 subjects you need to study. But if you study enough, they all are relatively "easy"
The QB does help, but understanding is of importance!