Assuming I can learn whatever and I have respected teaching credentials
I'm thinking LSAT, but perhaps the subject with the highest ceiling would be prepping high schoolers of the ultra-wealthy? I'm not sure. Perhaps Stack Exchange knows.
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I have worked as a private tutor for virtually all math, physics, and chemistry subjects, as well as a tutor for all major standardized entrance tests. My personal experience was that of the non-mathematical subjects, MCAT tutoring was the highest breadwinner. However, this took place in a town with a major university and a medical school.
I agree that prepping high schoolers of the ultra-wealthy is a major breadwinner as well, and may even out-compete MCAT tutoring depending on where you are at. However, your proximity to a neighborhood full of wealthy families (and the size of that neighborhood) plays a very large role in determining how much a tutor would earn. Furthermore, most of the ultra-wealthy parents are usually only willing to pay top-dollar for someone who is or was a high school teacher of the exact subject you're tutoring, or has a similar credential to back it up.
On a related note, I should add that although I've met some tutors who made quite a bit of money given the circumstances, most tutors do not consider their work to be a sustainable income.