It's all dependent, isn't it, on what you're good at? I'd personally consider Physics or Chemistry to be the hardest 'A' levels based on people I know who do them and the work I've seen them do - I did well in both at GCSE but wasn't interested in pursuing science. But someone who loves Science may well find English harder if they're not that way inclined, lol.
Out of my own subjects, Spanish is probably the hardest. Overall, Chem and Physics, Maths and Further Maths, and History on the humanities side because it's so content heavy.