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.
9/11 for sure makes a lot of sense. But I think it’s what followed it that marks the break. The Iraq war and the lack of WMDs and all that that implies and all the tremors that set forth through the democracies of the time. The nationalistic and racial energies drummed up. Conspiracy theories etc.
Along with YouTube breaking through into the mainstream and Time magazine putting a mirror (IE “you”) as its person of the year, I’d say 2006, also in part because of Colbert’s evisceration of Bush at the White House correspondents dinner.