A lot of it, I think, depends upon how your mind works.
Personally I enjoy anything relating to calculations and understanding how stuff works / problem solving. So, for me, PofF, Nav, P&P, M&B were all just good fun.
On the other hand, being required to use my memory a lot I struggle with - so Law and Operational Procedures were incredibly tough.
Met was somewhere in the middle - half memorisation, half understanding.
But, I know a chap who was an accountant by first profession (as you'll probably guess, I'm an Engineer) and so used to working with regulations - he found law very easy but really struggled with anything related to PofF and Nav.
G
(Disclaimer, I did CPL, not ATPL).