Second vote for the Battle of Britain. If that had ended differently then nothing would be the same.
This is a bit off the topic but would it have been any different? Hitler would still have had a great deal of difficulty in mounting an invasion as the RN was still very strong. Even if they had invaded, the logistics of supplying the occupying army would have been tricky.
I'm not denying the Battle of Britain was a heroic event and certainly hampered the German war effort but beyond being a morale booster (which is not to be sniffed at) was it really that significant?