Battle of Britain for World War II, no doubt.
A successful invasion of England would probably have left the USA out of the war and allowed Japan Access to Empire assets almost unchecked.
However, D-Day is the most important event for the post war. D-Day, as an operation did not really change the result of the war. Germany would have been beaten back the the Soviet Union, it may have taken longed, but it was looking inevitable.
However, having Empire and US forces in Western Europe at the end of the war stopped Stalin from creating a full communist europe, and probably won the Cold War for the USA in the long run.