Gen. Nav and Met were the hardest subjects for me to master. As above, if you are capable of memorising large swathes of information, Systems and Air Law, as well as IFR and VFR Comms shouldn't be an issue.
Gen. Nav itself was as much about understanding the concepts inside and out, as it was finding time to revise it sufficiently, certainly when the concepts used in that subject are much removed from aviation on the line today. Whereas, subjects like IFR Comms and Air Law for me were much easier, given their pertinence to flight - I don't go a day without referencing something from those two subjects.
Hardest subject is: Gen. Nav
Second Hardest is: Instruments & Navigation
Third Hardest is: Flight Planning & Performance