oKAY So, my opinions on those four exact advanced highers.
Maths - quite easy, definitely the one I do the least amount of work for. A bit boring but a decent 7/10.
Mechanics - hard, you need to put the effort in. If you think you can put lots of effort into mechanics whilst doing the other three then go for it. A bit boring if your teacher is a balding old man but I love the dude and the subject is really interesting. You have to be good at problem solving and just gritting your teeth through the questions. 9/10 for subject, 4/10 for the exams a bit long-winded, 7/10 for questions tho when ur not burnt out after 3 hours.
Physics - decent subject, am studying it at uni. AH is a bit (read very) dry. Mechanics definitely helps with the course, end up only really teaching myself quanta/waves and the theory bits. 5/10 at AH.
Chemistry - I have a really bad teacher who can't teach and getting through it, so I believe you can too. I'd give it a 4/10.