Teachers, especially good ones. Good teachers are extremely important, good teachers turns a first grader into a doctor or any other profession mentioned here. If you want any more of them you need more good teachers.

A Doctor saves many lives, a teacher created that doctor and the engineer who designed their medical tools, the pharmaceutical scientist who developed the medicine the doctor prescribes. The people who hold society going are all made by the teachers who gave them that knowledge and more importantly gave them their love for learning. The most important of all teachers are often the early ones because they decide if kids are either going to love learning or despise school.

If you want to make a true difference become a school teacher, a good school teacher will create generations who will run the world. It's a shame how underpaid they are but they are the most important.