Culloden.
The complete end to the Jacobite hopes of restoring the Stewart family to the thrones of England and Scotland. Which ended any chance of the Union being dissolved which paved the way for Scotland's industrial, agricultural and intellectual revolutions.
As a result the British empire became massive and the world got a multiude of great stuff from the minds of Scots.
The Battle of Kursk 1943 was big success for the Soviets, Hitler gambled everything on this one battle and lost, he also got it wrong thinking the battle took a heavy toll on the Soviets and they could not go over to offensive operations. Hitler was proved wrong when the Germans ran out of steam.
Another major battle was Operation Bagration on the 23rd June1944 which saw the destruction of the German army group centre, annihilating 17 Wehrmacht divisions and over 50 other divisions. This one battle inflicted the greatest defeat on the German army in WW2.
Another Battle which was a lost cause but never the less should be mention was the Warsaw uprising 1944 by the Polish people who gave it there best.
Normandy.
"The Normandy landings were the first successful opposed landings across the English Channel for nine centuries. They were costly in terms of men, but the defeat inflicted on the Germans was one of the largest of the war. Strategically, the campaign led to the loss of the German position in most of France and the secure establishment of a major new front"
It helps that it drove a stake through the hearts of the German HQ's, creating turmoil withen the generals. Also, without it, the Allied forces wouldnt of been able to get to Germany.