The subjects i do are maths, chemistry, history and philosophy.
In terms of understanding the content, i would rank chemistry the hardest but maths the easiest, with philosophy probably second hardest and history third. Chemistry is often really difficult to understand, but once you understand the concept it can be really easy. Philosophy is often actually really hard to understand, and the essays are a bitch. History i'm not sure about, with one teacher it's the easiest thing ever, i'd say my easiest subject, but with the other one i don't have a clue what's going on, i guess he expects us to do a lot of the work ourselves through background reading etc which i can never be bothered to do. And maths, occasionally it is a bit confusing when you first start a topic, but generally i find it fairly easy and once you understand an equation it's just a case of slotting in numbers really.
In terms of the exams, i would say both of my essay subjects were a lot harder to revise for and to properly answer the question for. I'd again say that maths was easiest, and chemistry also fairly easy, particularly because a lot of it was just memorising equations. There's so much information to learn for essay subjects and then they only ask you a really specific question on a tiny bit of the course which is always really annoying. I spent at least 4x more time on my essay subjects during revision, and i still feel that i have done better in my science ones. For my chemistry exam, i learnt over half the content on the morning before the exam. You would never be able to do that for History.
So overall i'd say my essay subjects are probably harder, though chemistry can sometimes be a bit of a nightmare!