If we're talking strict philosophy, however, I think there are some with more clout. For example Kant constructed one of the most cohesive and consistent moral systems in philosophy (As an aside, an Austrian woman, Maria von Herbert, wrote to Kant asking for guidance after being dumped. Kant's prescription of "a pure moral sedative" didn't quite do the trick and she ended up committing suicide. This however should not be taken as an indictment of his overall system - this is the cold stone table of philosophy, not a psychiatrist's chair).
Aristotle would be my prime candidate. After all, the man practically invented the style of modern philosophy as well as, of particular interest to this writer, being the first to write about economics (I believe he invented the term).
I think the BBC's helpful summaries make my case quite simply.
Most modern socialist theories are drawn from his work but Karl Marx has had a wider influence touching on many areas of human thought and life such as politics, economics, philosophy, and literature.
More than Plato and Socrates Aristotle's brand of reason influenced the progress of Judaism, Islam and Christianity through thinkers such as Maimonides, Thomas Aquinas and Averroes.
I think saliency has got the better of a balanced decision. (Mariella Frostop chose Søren Kierkegaard if you were wondering. Anne Robinson chose Nietzsche)