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.