It depends what you're good at. I knew some A* students (STEM students) who still thought a comma was for "when you take a breath" so i think they would have found English pretty much impossible. Equally, I've met people who worked their *** off to get a C at GCSE maths but got a As in History and Classics A level.
I would say when speaking generally that its easier to get top grades in STEM subjects because they have much more objectivity. You are objectively right in maths and they cannot drop you marks. Compare this to History, it doesn't matter how good at History you are there will always be something related to the topic you're writing an essay on you didn't put in (people write 2000+ page books on the same thing) so its easy for the examiner to go "hmm, its good but its not 100% good".
Coursework subjects are nice because you have essentially unlimited time to do the coursework (Ok, it is limited but i refuse to accept you were working non-stop for 4 months) which means its easy to get grades well beyond what you'd get in an exam to bump you up. Doesn't make the content easy but it makes it easier to get decent grades.
Overall though, I think maths is the easiest A level to get an A/A* in because you can just do 100000000000 questions and even if you naturally suck at maths you'll be able to do well simply because of how repetitive the questions are. If you're unlucky and your year has a curveball question you would miss the A* but you can reliably get an A and I think almost anyone can.