The two best works to begin with are the Enquiry Concerning Human Understanding and the Dialogues Concerning Natural Religion; they are highly readable and contain most of his key insights in epistemology and philosophy of religion, respectively. The Treatise on Human Nature is much longer exposition of the same points of the Enquiry; it has many of Hume's greatest and deepest passages (including the famous one about deriving "ought" from "is") but it is quite more difficult for the casual reader.
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?