Firstly, I think most of us can agree that in almost every situation, Exponential Growth and Decay is piss easy and possibly just a filler to add more content to the course as it's not really harder than the 2 unit version.
I like Binomial theorem, permutations and combinations, because I am usually able to spatially visualise something. [Except the number of ways n people can sit around a table is (n-1)!. I can get it but it's really hard to visualise, other than to try and think of them as in space and not in the seats of at the table. I also hate the (Tk+1)/Tk thing.]
SMH is easy to remember, with basically just one general formula which can be expressed in sine or cosine [sinx=cos(90-x), but in radians of course]. Although it relies more on understanding than knowledge.
Applications of calc to the physical world is not too hard to apply, but a complete bitch to remember the 4 main formulae, as well as the whole converting function of time to function of position. It would probably help to learn how to derive the formulae, not just memorising it.
Circle geometry is ok if you start by doing the proofs, starting with the "angle at the centre is twice the angle at the circumference" and using it to prove many others. Just get a general understanding and look for key signs in questions. Although it's annoying because some questions are obvious and some are really hard to see where to start.
That leaves me with trigonometry. I think overall it is the hardest topic simply because of the depth to it. From the transformation rule to 't' formulae, 2 angle rules, double angle rules, etc. It all leads on to each other, which is good, but can also make a question hard to start.
I'm yet to do induction, so I'm not sure where that fits in with difficulty.