As an American, it has to be Septemer 11, 2001.
Y2K had been a mostly smooth transition, thanks to a lot of hardworking cowboys. But the terrorist attack and response to it were such a sharp break from the world we lived in before it, in a way that still dominates media, culture, and politics today.
So yeah, to me, all of AD 2000, and most of 2001, were still the old millenium, still the 90’s, and for me personally still mostly my childhood in many ways.
From memory that was year that Obama made fun of trump at the White House correspondents dinner which odd sometimes thought of as the moment he really decided to run.