I think that it is a three way tie; that it to say, Socrates, Plato and Aristotle each made significant contributions to Philosophy and related disciplines.
Socrates, appears to have been the first Westerner to have invented-(or at least pioneered), the art of public debate and epistemological inquiry-(the famous, "Socratic Method"). His student Plato, was a prolific and voluminous Writer whereby he produced numerous "Dialogues"-(with Socrates as the Protagonist) on a variety of topics. In a way, Plato, was the West's first Political Theorist-(i.e. "The Republic" and "The Laws"). Socrates had an informal school and Plato was his best pupil. However, Plato, was probably the first Westerner to have founded an "Academy"-(the most prestigious College in the Ancient West); and his best pupil, was Aristotle.
Of the three Philosophers, Aristotle, is arguably, the most influential and perhaps the most erudite. The sheer volume and diversity of Aristotle's works is encyclopedic in scope. Topics, such as Logic, Rhetoric, his Biological/Natural treatises, his early works on Psychology, Metaphysics and Cosmology, Physics-(well, he wasn't too swift in this area), Environmental Science/Meteorology, Literature/ Poetry, Ethics and Politics, were topics that were covered by Aristotle within a 20-25 year period. Aristotle also founded his own School called, The Lyceum and had also tutored Alexander The Great.
Now it is very likely that some of his Aristotle's works were a mixture of original lectures, as well as the ancient equivalent of Graduate school research papers. Unfortunately, nearly all of Aristotle's written works are lost and his surviving works were lectures that have been meticulously compiled-(and edited) over the centuries. Nevertheless, Aristotle-(and his likely team of Graduate level students), produced, within a 20-25 year period, some of the most influential and studied works of Western Civilization-(and World History).
While I have written complimentary things about Aristotle-(minus "The Physics"), the three all stars of Greek and Ancient Western Philosophy, Socrates, Plato and Aristotle, are essentially tied for having made significant contributions to Western Thought. Each of these Philosophers were equally impressive and unique in their own way.