For me it's Nietzsche by a wide margin. After Nietzsche, you can safely throw away most of your philosophy books. Nietzsche brought psychological insight that Western philosophy had never seen before. Nietzsche dared to deconstruct and challenge the foundations of Judeo-Christian morality, rationalism and liberalism. Nietzsche created potent memes like the Ubermensch, the Last Man, God is Dead, the Will to Power, and the coming of the strangest of all guests, Nihilism, that shape our intellectual discourse to this day. Nietzsche was perhaps the closest thing the West has seen to a prophet -- a dark Buddha or Antichrist who haunts the Western philosophical enterprise like a specter. Nietzsche has been ignored, misinterpreted and criticized relentlessly, but his challenges to the philosophers are as potent as ever.
These are my thoughts.
Nietzsche by a wide margin
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?