Pacific Theater: The Battle of Midway in the pacific due to the fact the Midway offered a strategic advantage to whichever side controls it. If the Japanese took midway, I think history would have played out very differently with a possible land invasion of Hawaii, and even mainland USA. Thanks to our intelligence network though, we were able to thwart the Japanese plan and take the fight to their country.
European Theater: In Europe, I would point towards the D-Day invasion because it was due to its success that German influence was reduced on both the Western and Eastern sides of Europe. Another example which works in tandem with the D-Day invasion. could be the failed invasion of Russia by Germany as it brought Russia into the war from the east and allowed the Allies to push the Germans back on both the eastern and western side into Germany and ultimately Berlin.
