Generally speaking, a decent rule of thumb is that the subjects that were around 30 years ago are the hardest (with the exception of computing). So no general studies or stuff like that.
But really, are you looking to do the hardest ones or are you looking for a combination that universities will respect? If you're going to apply for sciences at a top uni, you should be looking at taking 3 sciences & maths. Likewise, with an art or language subject they'll be looking for essay subjects.
What you should REALLY not do is take two essay subjects and two sciences. That way, whatever you apply for, it'll look like you don't really care.