There could be dozens of things mentioned in response to such a question. I will mention three.
- The Spanish conquest of Mexico and South America. This represents the overall trend toward European imperialism that had such a huge impact on the world. I would include African slavery was part of this impact.
- The Protestant Reformation. This was important because of the way it led to religious wars in Europe. It also can be credited (as Max Weber would) with helping to promote capitalism.
- The Industrial Revolution. This changed the way that everyone in the world lives as it has made us able to have access to more kinds of goods. It has also ended up "shrinking" the world and causing the globalization that is so important today.