Sextus Empiricus is an important candidate I don't see discussed on LW. I don't endorse his positive philosophy, but his positive philosophy wasn't really what had the biggest influence; it was the distrust of speculation and theorizing he helped inspire. Historically, I wouldn't be surprised to learn that Sextus' demolition of all the theories of the day had a larger impact on the growth of science than any particular positive epistemological doctrine.
Anselm is another good candidate, surprisingly. He made philosophy cool again after the age of darkness, inadvertently planting seeds of intellectual investigation and disputation that would cause Christian thought to burn itself out. And scholasticism wasn't a thing yet, so he didn't even have much obfuscatory systematicity on his side to slow the rot.
Since LessWrong is a major congregation point for certain philosophical ideas, and because people here tend to be more objective (in the sense of not being self-deluded) than elsewhere, I thought I'd ask people's views.
To be clear, by "Greatest Philosopher" I am referring not to the most correct philosopher in human history but the one who deserves the most credit for advancing human philosophy towards being more true.
Off the top of my head I would say that a prime candidate would be Hume- amongst other things he rejected the idea of a soul, realised to a much greater extent than his predecessors the limits of human knowledge, and opposed the idea that reason is somehow an objective force that can make priorities independent of emotions.
Aristotle deserves considerable credit relative for his time but doesn't make the list because although it wasn't his fault his ideas were dogmatically accepted and held back both science and philosophy later on.
Your thoughts?