What are the three greatest Greek philosophers and one contribution made by each?

The three most prominent Greek philosophers today, who have had the greatest direct influence on philosophy through the past 2500 or so centuries, are probably Socrates, Plato, and Aristotle. Nothing of Socrates' philosophy exists today in its original form, but Plato's writings often use Socrates as a character in a dialogue, so great debate exists even today about how much of Plato's ideas were his own and how much were Socrates'.