that's completely opinion based but AS wise i can confirm imo that physics despite me being very good maths wise is BY FAR harder.
I dropped biology after AS but i feel that if i did A2 biology with the A2 physics then physics would still be harder as biology is nicer maths wise and as long as you can remember the information you can do pretty well. In physics the maths questions are riddled with tricks, the practical section requires so much more than simply remembering the practicals, and the questions can be very decieving in physics.
again, id like to repeat I havent taken A2 biology so i dont know which is definitely harder but from my AS experience i know physics is a nightmare and i believe to be considered the hardest A level closely followed by further maths