The Logos is the thing. The single greatest thought ever thought.
It was that idea, more than any of the other pre-socratics, that for me was the singular greatest thought in all of human history. That the world is as it is by some inexorable reason, which can be uncovered by mind confronting matter. That it is not the magic or whims of gods and demons, or even the will of man or man-like things that makes water wet and sunlight warm. That the issues of change versus permanence, substance versus essence are all well and fine, but not the questions that count.
Heraclitus was da man.