Quite hard to say which is the hardest degree. For one, hardness varies from person to person; for example I find the sciences far easier than essay subjects, but some of my classmates are the exact opposite. Furthermore there are so many different factors that contribute to difficulty: volume of work, are the concepts abstract? etc.
You will get the common answers of maths and medicine being the hardest degrees, and they are definitely taxing degrees, but to say 'this degree is the hardest, period' is somewhat naive in my opinion. It would be better to say 'this is the hardest degree for me'.