I would argue and say that there is no "smartest" A-level. Sure, there are statistically smart subjects...there are subjects that are viewed as the smartest for their accreditation...but there can't be one.
If someone has a strong talent in Maths, then surely A-Level maths, with the right previous knowledge, would be easy? The same would go for every subject. I know plenty of people who do Art and Photography and ace it, and have done for as long as they've done art; simply because they are talented with Art. Whereas those like me, without much artistic-in-this-form skill, failed it even before GCSE.
I also believe that if you bring a child up in a certain way, you can give that child talent. 90% of a child's abilities, aspirations and opinions is down to upbringing. For example, the Chinese government bring up some children from a very young age to compete in the Olympics for example.
Just my 2p.