It depends on what you have an aptitude for. In terms of the hardest exam to do well in, I think languages have to be up there because you can be consistently good throughout the year but completely flop the exam for no apparent reason, and you might not even realise you did badly until your results come out. This has happened to me and other students at my school numerous times.
EDIT: Probably not the best time to ask this question actually, OP... Lots of people are going to be biased according to their results yesterday. If you got a bad result in one subject, evidently that one's going to be the hardest subject ever at A-level!