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.
I think that day changed a lot of things for everyone around the world, not just for America. Everything that came after felt very different. 9-11 shaped our society and media in a way we can still see today.
The 90s were a pretty optimistic decade up until the year 2000. 2006 and 2009 were great individual years regardless.