While grades and performance come into it a bit, my advice is to choose A-levels based on what you want to do in the future. It's all well and good getting A*s but if you do all languages then decide you want to do Biochemistry at university, you're stuck as most, if not all, universities require specific science or maths A-levels.
You have to make sure you're going to enjoy what you're doing otherwise there's no point, but please have a think about what you want to do or study beyond A-levels and what that will require, rather than what you think you'll get A*s in. The sciences are hard, I'm not going to lie. I'm in A2 now doing biology, chemistry and physics and it is no easy walk. But it IS do-able and if I can do it, you can. Maths was one of my weakest subjects at school but I came out with a B at AS. It is still possible to do well, just means you'll have to put that extra effort in if you want those elusive top grades.