I suppose I mention Socrates because he was so important in developing maybe the first system of critical thinking. There is no doubt that great thinkers existed before him, Pythagoras as you mention for example, but we are into the realm of maths here and not science with its emphasis on empirical data from observation etc. I'm talking about a systematic methodology for arriving at conclusions...that dialectic model is very much part of modern science too...the whole Hypothesis/Antithesis/Thesis thing. Popper's whole science and falsification ideas are pretty much a direct descendent of the Socratic method. Of course, Eastern philosophy is a whole different ball game but its a little too metaphysical for me.
By the way, are you familiar with Zeno of Elea and his paradoxes...a contemporary of Socrates as it happens? They can be fun and a great example of the dichotomy between the world of thought and the physical world...whatever that actually is, some real strange ideas about that now we are fully immersed in this digital/quantum stuff. Too late in life for me to get into all that...I'm more at home with my Hume, Kant, Spinoza and the rest of the oldies from that Monty Python song!