In terms of understanding concepts, I'd say Chemistry is hardest. However, History is also quite hard in the amount of time you have to dedicate to extra reading and planning and writing essays. There's also a lot to remember! Lots of work but I'm enjoying them so far, let's hope it stays that way!
Its the fact that a lot of teaching contains reactions and mechanisms, but just a few lessons are relate to practical things like vulcanisation to name an example.
MATHS- hate it! im such a flop at maths because i find it difficult to concentrate when the teacher is talking, plus there's only 4 people doing A2 maths so you can't get away with not bothering to work/not doing hwks, so Im always in the spotlight