Yeah, I was thinking the wars between Greece and Persia. If Persia would have conquered Greece and stamped out its culture, then our culture today would be vastly different.
And as for the religion generally associated with Western Civilization, Christianity would not have been able to spread so far, had it not been for Alexander the Great's conquests unifying the known world.
Now, what would be the most important battle?
I believe the Battle of Salamis would be, though it is a close race in my book with the Battle of Marathon.