10k - 2k BC Humanity advanced so much during this period, Egypt, Iraq, India, China, etc. that it surprises humans as modern as of today (who consider Aliens helped them achieve such feat).

I also want to know what went wrong that the whole world went back to being illiterate villagers soon after that. (I'm betting my money on climate change).

My second choice is 2k BC - 0 because of similar reasons as mentioned by @slolean13