On those criteria, I would say Plato. Because Plato came up with a whole mess of ideas that were... well, compelling but obviously mistaken. Much of Western Philosophy can be put in terms of people wrestling with Plato and trying to show just why he is wrong. (Much of the rest is wrestling with Aristotle and trying to show why HE is wrong... but then, one can put Aristotle into the camp of "people trying to show why Plato is wrong.")
There's a certain sort of person who is most easily aroused from inertia when someone else says something so blatantly, utterly false that they want to pull their hair out. Plato helped motivate these people a lot.
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?