Kilbaha is always my choice for a tough paper, but balance it out, I'd suggest doing something like Heffernan or MAV because it's probably more reflective of what VCAA exams are like.
Note that doing "hard" exams might not always be the best strategy because the way one approaches a hard exam is different to an easy exam. In hard exams, it's a real test of your ability, so you are trying to maximise the number of marks you can gain, whereas on easy exams, it is a test of your meticulousness and care, so in a sense, you are trying to minimise the number of marks you lose.
These are two different exam strategies, thus you should balance your preparation out between medium, easy and hard papers, you can do 100 hard papers, but if you don't have your pace and care aren't up to scratch, you can still lose marks on the easiest VCAA exam ever should you be faced with that on the day
