Which career most accurately describes someone who would be classified as a thinker?

The correct answer is - philosopher.

The career in which someone will be considered as a thinker is a philosopher. The main and most recognizable trait of a philosopher is the thinking. It can be about all sort of stuff, be it the existence and the meaning of it, a social behavior and how it should be or what's wrong with it, even entering the sphere of the natural sciences and thinking about the natural phenomenons. This profession is very often controversial, and it is rare that the actual opinions of the philosophers are accepted, but as it has been the case for thousands of years, it is usually after their death when their thinking and opinion are given the recognition they deserve and are accepted by the wider masses.