So 1492 has to be at the top of near the top. There are debates a plenty about this in other threads. But IMO it has to be one the most important dates in human history it directly affected massively a huge swath of the globe and indirectly almost the entire globe. Plus it has an actual precise date where the process began unlike most other revolutionary changes in human history. To be clear it was absolutely a bad thing (even if some of the indirect effects were not necessarily bad, I’m quite comfortable calling it a “bad thing”), more like the Nazi occupation of Europe than the discovery of penicillin. That doesn’t make it any less important.
Talking of which I think the discovery of penicillin counts. It utterly revolutionized human life like almost nothing else, and (unlike most other discoveries which are the product of decades or centuries of iteration) it can be traced directly to Alexander Flemings petri dishes in 1928 even if it took many years for it to change anyone’s life