I think it's mathematics. Some people say it's easy, but in my experience these are the same people who just breeze through it, regurgitating formulas and never really understanding them. In that regard, yes, mathematics can be easy. But on the other hand, that's not really mathematics, that's just repeating what you're told. Taking apart formulas and understanding the exact logical mechanisms by which they work is not easy and certainly never considered within the narrow and rushed scope of a typical university course. I remember in first year linear algebra I literally went door to door through the entire mathematics faculty trying to figure out how exactly the rotation matrix turns a vector. Only one person in the entire building could answer this question.

That's why I consider mathematics the hardest thing to study. It can be easy to do. But it's extremely difficult to comprehend, particularly at the more advanced levels.

Foreign languages are also worth a notable mention. But as a multilingual I just want to state that foreign languages are not intrinsically difficult to learn. The problem is that in the Western world grammar education is extremely poorly taught (if at all), making learning a foreign language fluently next to impossible for a Westerner. I have only seen this happen extremely rarely. And this is not a slight against Westerners, but it's not surprising. If you don't understand your mother tongue well, there is absolutely no point in trying to learn a second language. However, once grammar has been mastered, learning a foreign language is not particularly difficult.

I would love to hear your opinions on this question.

I will be back in 4 hours to answer any comments and look forward to reading your opinions.