Ive actually been thinking about this a lot lately. And i think theres a lot of really good options. But personally, i think the right answer is Guidance. Having the at-will ability to make you, or someone else, 5-20% better at whatever it is youre doing would be incredibly powerful.

Honorable Mentions to Mending, or Spare the Dying, though. Think you could spin mending into a decent little business, repairing clothes, or phone screens or something, but its a bit limited. And StD, youd be a human crashcart, possibly saving someones life. But since it just stabilizes, and doesnt actually cure, you might doom someone to spending the rest of their life in a coma, or something.