Speculum Criticum wrote in with a comment:
But the really interesting question, i.m.o., would be, Who is the most *under*-rated philosopher? The Over-rated question is just an invitation to polemic. The Under-rated question could bring all sorts of interesting neglected figures to light, & possibly be quite fruitful.
Such begins the split-off from the original question Harman raised about the most overrated philosophers. I was thinking of this question when he asked about the most overrated, but I’m not even sure how to begin answering the questions. (1) You could name the most slandered philosophers who is clearly influential and for good reason. Hegel comes to mind, since for so long he was treated as proto-totalitarian (he thought of a System and you know where that goes…), but is crucial to understanding any of the main figures of the past one hundred years in Continental; or (2) there’s the philosopher whom no one except you and your reading group over at the Rotary Club have heard of, who would change the intellectual landscape if anyone but you was reading him or her.
More interesting would be the most underrated books, and this is where there’s a good pedagogic moment for all of us is available: what work has really changed the way you look at philosophy, yet few people pick it up to read it?