What is the most important Event in European History

[Hurin_Rules]

I would have to say the Fall of Rome.

It meant Europe would not be another China; it gave rise to multiple new kingdoms that competed with one another; it gave Europe a written heritage but did not dictate how Europeans interpreted it; it gave rise to struggles between Church and State; it undermined slavery and reoriented the European economy northwards; it transformed social relations, providing fertile ground for everything from feudalism to city-states; it united Germanic vigor and concepts of personal freedom with Greco-Roman language and learning; it not only allowed a new Europe to be born (in the age of Charlemagne and, much more strongly, in the 11th/12th centuries) but also gave it its educaton.

P.S. Can't wait for Barbarian Invasions!

P. P. S. Honourable mention goes to the Industrial Revolution.