Its all dependent on what your strengths and weaknesses are
but if I attempted for example Further Maths...or Spanish/French/German I don't think I'd last very long. For me languages and maths based subjects are the most difficult. But its all subjective
There's no outright toughest tbh, depends on what kind of student/person you are. Maths is difficult. Unless you do a lot of work then languages are very hard to get decent grades in, based on how some of the people in my class are doing this year anyway. (AS)
The highest rate of A* goes to maths, further maths and physics. If you are mathematically minded, those subjects are the easiest for you to do well in. If you are not, they would probably be the hardest subjects to do well in.
Have to agree with German being tough. Any modern foreign language really; I really enjoy doing German but the fact is at A-Level it's a lot more about miniscule focus on grammar points; in comparison to GCSEs where it's a hell of a lot less focused.
The sciences- there is so much to learn! Also Spanish is hard. I don't do maths but there is a reason why I don't, at GCSE I quite enjoyed it, but A level it would be a different story! :-)
Toughest A level is joint between Physics & Additional further maths. Further maths is arguably only "hard" if you do the mechanics modules whereas if you were to study statistics it is largely more straightforward and easier to comprehend.
So it depends on what you do. ^
Although, I do agree "difficulty" is largely subjective.
Any modern foreign language. Further maths (mechanics) and physics. Chemistry isn't too bad (though I'm biased) but I can see why people will think it's hard.