(Doing ASs)
From easiest to hardest:
Maths, Chemistry, Physics + Biology the same, though maybe Physics before biology because the mark scheme is less of a bitch.
Maths I hardly put any time into at all, I only do past papers as revision. Chemistry is just a little bit harder than maths, I find I grasp the concepts quickly so most of my revision is aimed at factual recall rather than working on the understanding. Physics a little more difficult and actually spent so much more time on it than the previous two, similar to Biology.
But to be frank, I don't find any of my subjects terribly difficult.