Arguably, the easiest are Media Studies, Photography, Sociology, Psychology, anything else like that, where you can basically rote learn a list of facts/studies.
Hardest: chemistry, physics, maths and further maths (conceptually hard), English Literature and History ('hard' in the sense that you have to read a lot, and then evaluate what you've read).
I didn't take any sciences, but I think it's generally accepted that maths/physics at an advanced level is difficult.