Although, if more was known about Harappa and such, it would probably be the ancient Indus River Valley civilization.
Egyptian society is still, in fact, the model for a perfect society. No society today even comes close to rivaling how perfect Egypt was in the Middle Kingdom era.
Sure, there were slaves in both of these civilizations, but they weren't slaves unwillingly. No, in fact, most of them enjoyed their (often, lowly) position and jobs, at least until the later revolts and decay (in both).
They didn't give in to mercantilism (-esque anyways, there weren't what we would consider formal systems of economics back then) and stupid rivalries like the Mesopotamians or Jews. No, they remained true to their divinely-mandated culture that was brought by the first pharaoh Menes.
A thoroughly theocratic and despotic government. Which is why their great monuments still stand today. The longevity of their civilization is only surpassed (depending on your definition) by the Chinese.