Well, BBC listeners chose, surprisingly, Karl Marx by an overwhelming majority. The next three in the runnings also exercised great influence for what most would call evil: David Hume, Ludwig Wittgenstein, and Frederich Nietzche. Finally, Plato gets in there, but his 6% pales in comparison to the 28% garnered for Marx.
I'm interested in what Mere-O readers think of this poll and its results (Is Marx even a proper philosopher?). Also, who would you name as the greatest philosopher and why? Take a look at the BBC shortlist to give yourself an idea of the criteria for a "philosopher" - you know it is broad when the likes of Kierkegaard and Marx are on there.