... it's also about the combination of a-levels (if you did maths, further maths, and then travel and tourism as a third, you would possibly be disadvantaged next to someone who did maths, further maths and physics, even though you had two subjects in common), the grades you get (an E-grade maths student is probably not more employable than a BTEC student who gets distinctions, even in a 'less profitable' area) and what you do with them. I don't think there's any a-level which, in isolation, is going to mean you'll definitely do better than someone studying some other a-level.
If pushed, I'd say additional further maths - because you have to do regular maths and further maths first, meaning you'd have 3 'traditional' and respected A2s, and you'd have to be very good at maths to be entered for it, so you'd probably have very good grades in maths and further maths already. But it's not that simple on the whole.