Nav seems to trouble a lot of people... It is just maths and there is only one right answer...
You've also got to be able to draw accurate lines on a chart from and to the correct places and then measure those lines accurately. If you go wrong at the start, it will carry through in all your answers. Personally I found Nav a doddle and would rank it among the easiest along with HPL and Comms.
Air law is the most boring, but it's only half a book and really not that hard. Meteorology (also half a book) however is a bit of a bitch. I still have no clue what the CAA accepted method is for calculating surface winds for a given time of day, and none of the instructors I asked had a clue either!
the most difficult imo? met. utterly boring - learning about clouds and precipitation.
It isn't hard and I didn't find it boring but it certainly isn't as straight forward as it could be. It gets my vote for the most difficult.