Plato

Who he?
Plato is widely considered to be one of the most important members in the foundation of modern western philosophy and had a great influence on those to come. He founded the Athens Academy, which is where you went if you were brainy.

He sits somewhere in the middle of the influential greeks, as a young man, one of his more formative experiences was meeting Socrates, who instilled in him the desire to know more stuff. While he briefly considered a career in politics, like many in his family lineage, the execution of Socrates for the crime of being a troll gave him a hefty enough dose of skepticism for authority to turn him into an influential philosopher. Later, Plato would teach Aristotle, as such, the affect he would have had on Virtue Ethics is probably more considerable than he is given credit for.

Fun fact: there is speculation that 'Plato' was not actually his birth name, rather a moniker that related to his rather broad build, so all of you who've referred to him as such have been continuing one of the longest pieces of name-calling in history (over 1500 years, go team!).

Wider reading:

 * Here is the Harry Hiker intro quiz on Plato
 * The Republic, one of Plato's more influential surviving works