Tyler Cowen asks a mostly stupid question and gets mostly stupid answers: What career helps other people the most?
I feel that many people ask this question and settle on a field with linear benefit. I hesitate to make such a sweeping generalization... but in my experience, women have been more likely to take such questions seriously, and more likely to end come up with the wrong (aka "linear") answer.
By "linear" here, I mean a job where productivity is nearly linearly associated with effort. A nurse can handle X patients per hour, after that you need another nurse. Basically, if the job can fit the starfish parable, it's linear. If the job is more like "guy who designed the circuit board that improved the efficiency of the starfish saving machine by 40%," it's not linear.
Possibly the best non-linear suggestion among the comments at MR was "CEO who sends jobs to developing countries." Such CEOs construct physical infrastructure in developing countries. They pressure local governments into good long-term policies like avoiding inflation, respecting private property, maintaining order and the like. Most of all, they contribute to the development of human capital in the developing nation.
It's worth noting that this applies mostly to manufacturing and design jobs such as those done in India and China rather than resource extraction jobs such as those done in Africa. See resource curses.
It's also worth noting that India and, yes, China, have governments that permit this sort of investment, whereas many South American and African nations do not. The differences in outcomes for those countries suggest that politicians also have a tremendous multiplying effect. Consider Venezuela or Zimbabwe.
No fortune 500 CEO is in the 90th percentile for businessmen, and no president or cabinet minister is in the 90th percentile for politicians. Still, even at a lower level of success, the amount of good that one could do vastly dwarfs any of the linear responses from the article.
For those who don't dig business or law, it's probably best to find any sort of work that will increase the efficiency of those who gallantly take on the linear jobs, one starfish at a time.
Failing that... quit asking stupid questions, get the highest-paying job you can find, live on as little as you can afford, and donate the rest to the starfish-saver of your choice.
Failing that, admit to yourself that "maximizing the help you do for others" isn't the most important guiding principle for you. =)
As a side note, I'd love to sit down for a chat with the fellow who suggested "US Marine."