I don't think any of the PPL exams are difficult, but I perhaps remember the one I had to spend a bit more time on was Met. On the other hand some are really easy (Human Perf, Comms) and if you have a little bit of all round aviation knowledge you could probably attempt the test without having opened the book (Not that would be reccommended practice, mind you)
On the other hand the trickiest I found was Nav, for the simple reason that if you get something wrong at the start of your planning it will carry forward and mess up the whole thing, even if your reasoning is right.