Mmmm…good one. I guess it would have to be science, most of the analytical techniques and decision making processes I use in my work have their foundations in what I learnt through physics and chemistry (although I was never that good at Chemistry!). As a safety engineer you need to have a questioning nature and be prepared to do a fair bit of research to understand how you can claim that something is safe, so any subject that makes you question, research and delve a little deeper will all help.
I found my maths and physics (the resolving forces) are still things I use in my current job. A lot of my work comes down to force diagrams just like I did in A level physics. The understanding of the fundamental basics is essential to my more complex aspects.
However, I also have to understand the importance of the world I work in. So having business understanding through business studies or economics.