Jesus. And I don't mean that in some pious "Sunday School" way either. Jesus truly was the greatest philosopher to have ever walked the earth.
Socrates was wonderful, but he attempted to extrapolate the truth. Even Buddha had many good teachings, but he pointed to a truth. Jesus IS the Truth; He did not point to any abstract truth, He did not teach that we needed secret knowledge, instead He pointed to Himself and showed that He was the foundation for all metaphysics, all epistemology, all ethics, all government, and all things involving philosophy. Though Jesus is so much more than a philosopher, He is certainly the greatest philosopher to have ever walked the earth.
All others either work against Him and are therefore wrong, or work for Him and are therefore below Him.