I gotta go with the Berlin Wall coming down. It changed the whole shape of my internal world, which may not mean much to everyboyd else but sure makes it memorable to me.
The fall of the Berlin Wall
The fall of Vietnam
The shooting of Ronald Reagan
The Challenger explosion
Who shot J.R.? (I know, this doesn’t even deserve to be on the list, but in terms of sheer frenzy at the time, it captured public imagination even more than the death of Princess Diana)