I will take a shot at this question, though with great hesitancy. The three philosophers that pop to mind are Socrates, Plato, and Aristotle. And because you are requesting a single philosopher, I would place a small wager on Plato for it is through Plato that we learn the most about Socrates (there are other sources, but Plato's is the richest, I suggest) and Plato was Aristotle's teacher (for about 20 years). A great 20th century British philosopher once observed that the history of philosophy is the history of footnotes on Plato. That, of course, over-states things and perhaps sacrifices accuracy to wit.
In good conscience, I must confess that I understand myself as a Platonist --so it may be that I am not the most objective in such matters. Although A.E. Taylor's work is a bit dated, I highly recommend his work on Plato --very accessible and puts Plato in context.