Plato (427 BC - 347 BC) is perhaps the single most influential philosopher in history, his only rivals for this position being Aristotle and Kant.

Like his teacher Socrates, Plato began by questioning everything (including Socrates himself). However, unlike Socrates, Plato began advancing concrete philosophical positions of his own. He developed the first comprehensive philosophic system, covering everything from the nature of reality to a very specific account of the ideal state (“The Republic”), in which justice reigns supreme. Plato’s distinct characteristic was an insistence of carrying each chain of reasoning to its necessary logical conclusion, so his totalitarian outlook in politics was the inevitable result of his two-world dichotomy in metaphysics.

Plato has been, and may, with some justice, be called “extremist”, “evil” and even “fascist”. But I have never heard of Plato being described as unimportant or uninteresting.