This question is, of course, a matter of opinion. So, I am sure that there will be many good answers. In my opinion, the greatest impact so far in the 21st century is the financial crash 2008. There are three reasons for this.
First, I believe that 2008 was just the beginning. The economic problems are not finished and there is very little in the world that is going well in terms of the economy that it will be very difficult for the United States to get out of this mess. In short, I do not see a catalyst for the economy to pick up. Stagnation is the new normal.
Second, the economic trouble of the West show that there is a shift taking place. For example, western countries are by and large debtor nations, whereas Asian countries are creditor nations. Historically speaking power goes where the money is and the money is in places like China. So, we are seeing a shift of power right before our eyes.
Third, the economic problems in America also shows great domestic problems. In a word, there is much greed on all levels. For instance, the House and Senate should not be able to trade stocks, because they have so much sensitive information. It really is akin to insider trading, but they do not do anything to fix this problem. Also the banks, which were too big to fail, are now even bigger! Also all this printing of money and lack of jobs point to stagflation.
It is important to keep in mind that from a historical perspective, debt and the lack of jobs create great social unrest, which can lead to greater problems.