The advice in this thread has pretty much covered everything I would have otherwise said, but, make sure you choose subjects you would be happy about studying.
Ask yourself, could you stay up in the early hours of the morning pouring through history books with a highlighter and post-its? Would you prefer practicing physics formulas or learning chemical definitions/formulas etc. If you prefer studying history, it goes without saying that you will probably study it more, be paying more attention in class etc. and subsequently you should do better overall in the subject.
Scaling doesn't affect you negatively if you come 1st in your class at your school. Don't pick things you hate, simply because it scales well. If you like the subject, scaling is an added bonus.
Look at the subject syllabus itself, look at what you will be learning and check out some of the past papers for the subject, so you know what types of questions/exercises you will be doing.
Hope I helped, good luck with your choice