Probably WWII
It marked the beginning of the US being in a charge of global security [which was opposed by Soviet Union]. As result the US spawned the UN and fundementally changed international relations. Also the end of WWII resulted in Japan leaving China and ending European interfere with China. Also profoundly changed Europe and it's colonial relationship with rest of the world.
Considering the destructive power of modern weapons of war as demonstrated in WWII, it could said that all wars since has been on the scale of police actions or border skirmishes- and that we have had mostly had global peace for last 50+ years.

The attempt by the Soviet Union to defeat the US [the Cold war which was a direct result of WWII] has had huge affect upon the world and upon the US.

One could argue that the American revolt from British rule and the French Revolution follow by the Napoleon wars had more effect- that it set the stage for WWII- that WWII was mostly an echo from it.

But one could argue that the most good came from WWII- had not the US won that war, the world would have had in short order, nuclear weapons or leaving out nuclear, massive capabilities in terms modern weapons of war- airplanes, machine guns, land mines, chemical weapons, long range missiles, etc. And of course the industrial technological capability to make all of this stuff.
Including nuclear weapon the world had a fairly easy way of ending global civilization by war. That was never the case in the past.