The Boston tea party, of december 16th, 1773.
This was a decisive military action against the English, which basically was the start of the United States as we know it.
As the US is now leading the Western world, I believe we can clearly state that this battle has had a great impact on the world as we know it, and on a lot of the events that have taken place since on world scale.
How would WWII have finished, if the Boston tea party had never taken place?
Even the battle of Waterloo (I am currently about 15km away from Waterloo (Brussels, Belgium) while writing this btw ;-) ) would've been different, or not have taken place, if the Boston tea party would not have happened (the army of the British Duke of Wellington was at about half force when going to battle against Napoleon, because a large part of his army was still in the US, or had died there).
Quite possibly the Battle of Vienna on September 12 in 1683. Vienna was surrounded by the forces of the Osman Empire and defended by forces of the Holy Roman-German Empire, Poland-Lithuania, the Habsburgs and Valakia. The defenders of Vienna were originally about 16,000 strong and the Osman army was 90,000 strong. The relief army was led by the John Sobieski, King of Poland, and was 80,000 strong. The Osmans were defeated and driven out of the heartlands of Europe. Their victory may have been followed by Islamization and centuries of oppression. We only need to look at the state of the Balkan to see the hatred and resentment Osman rule left in its wake. Europe may have been pushed back culturally and the strict doctrine of Islam may have stifled intellectual and scientific development in Europe.
I would say World War 2 and the reason why is that the world was threatened by one countries dominance. Germany wanted to rule the world and they came mighty close to achieving it. Europe fell too quickly and weren't prepared. If it wasn't for the Russians , English and the Americans the world would be a total different place today.
There are many important battles through the ages but this one stands out for me as it was crucial for the world powers to come together. This will make any country think now that they arent fighting one country but others will get involved. keeping world peace is crucial to the stability of the world.
Since @bozz took my WW@ answer I'll go a different route
Think about what the world was like then and what would have happened if Napoleon had won at Waterloo.
Would Germany even exist? What would have happened after Napoleon died a few years later of stomach cancer and his 13 year old nephew Napoleon III would have assumed the throne.? Most likely a civil war would have ensued.
What would the world be like now with 200 years of chain reaction/alternate timeline from that single event and what followed?
Baguette and cheese anyone?