I can't decide between "Just do it" (Nike) or "A diamond is forever" (DeBeers) . Both are great ads.
As for the momentous moments in life, Most of the ones I've chosen are within the last decade: Diana's death, Berlin Wall, 9/11. I suppose in terms of most memorable (what were you doing when?), I can remember the emotions that went through me during 9/11 more than the others. Although Diana's death comes close.