I think its the moon landing, who doesnt know about it? How many peoples perspective of the universe we live in was changed forever? I think it really changed the way we view ourselves (hasnt stopped us killing eachother mindyou but I said it was signifigant, not a mirracle
).
Though that was a positive thing and we all know that its only really bad things that linger with the public mind so I would say 911, Its not the first time so many have died at once but I think it was the first time anything like it was televised live, everyone worldwide had upto the minute updates on the situation, I think I only met one person who didnt know about it during the first couple of hours and I havent met anyone since who didnt know about it.