Harder in what sense?
There are some subjects which are easier to get an A in, but much more difficult to get an A* in.
In some subjects it comes more down to working hard, whereas in others if you're not good enough you will struggle.
Four different measures of difficulty, which produce wildly different results.
If we're talking work and talent required to achieve an A*, I'd say languages/music/art.
If we're talking hard work required to get an A, then it might be a subject with a heavy coursework focus e.g. Engineering, or perhaps subjects with a lot of content like History/Biology.
Just picking the "most difficult" subject is pretty uninformative and depends entirely on the type of student.