That's a question from Katja Grace. Let's assume pure marginalist act utilitarianism, namely that you choose a career and get moral credit only for the net change caused by your selection. Furthermore, I'll rule out "become a billionaire and give away all your money" or "cure cancer" by postulating that said person ends up at the 90th percentile of achievement in the specified field but no higher.
What first comes to mind is "honest General Practitioner who has read Robin Hanson on medicine." If other countries are fair game, let's send that GP to Africa. No matter what the locale, you help some people live and good outcomes do not require remarkable expertise. There is a shortage of GPs in many locales, so you make specialists more productive as well. Public health and sanitation may save more lives than medicine, but the addition of a single public health worker may well have a smaller marginal impact, given the greater importance of upfront costs in that field.
An important question is whether the said job candidate should be seen as precommitting to an honest disposition or whether we should treat the person as developing the median disposition, in the chosen career field, over time.
What do you all think? What other career — at the margin — has the stongest positive effect on other people?