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!
Chemistry without a doubt. There's so much to learn, and it isn't exactly conceptually easy. I found physics easier just because there was so much less content to crawl through, but I think it also helped that I was interested in physics more. I think if you're interested in a subject you'll want to work harder, therefore you'll do better. But of course the problem is we don't know what subjects you'd find tricky OP because everyone is different and responds differently to some courses.
I'd say English Lit and Maths. Chemistry's also insanely difficult from what I've heard. I could hardly handle GCSE . Maths and English could either be the easiest or most difficult subjects depending on whether you have an aptitude for them or not. Those who do, find them piss easy, and those who don't find them an absolute nightmare.