Well, for me it would be the end of the Cold War. That was a momentous time for both international politics and the life of my Church. Personally, the single most important event within the overall end of European communism was the December revolution in Romania. Had that not happened I'd never have met my wife, never have had my children and, quite conceivably, never have become Orthodox either. Without the fall of communism, I'd almost certainly have never met the majority of my family either as they were separated from us by the Iron Curtain, being mostly Czechs and Slovaks. I'd say that what a person considers most historically significant is the event that has the greatest impact upon them and their family, and as the end of the Cold War had both personal and general effects of great importance I'd choose that every time.
James