Disclaimer: I know how Chaos Theory works, and I know how changing a single historical event can change literally every single thing that ever occurs after it, but let's ignore that for the purposes of this thread.
For me, it would either be the Battle of Tours (http://en.wikipedia.org/wiki/Battle_of_Tours) or the Final Siege of Venice and the Battle of Lepento (http://en.wikipedia.org/wiki/Battle_of_Lepanto).
If the Umayyads had won the Battle of Tours, we would probably be ~400 years ahead in terms of scientific progression right now since the scientific prowess of Andalusia would've been unleashed on Western Europe a full ~500 years before it actually managed to get through (which, when it did, it triggered the Renaissance.) The Dark Ages/Medieval Period would've been cut short. Enough said.
If the Ottomans had won the Battle of Lepento/the Final Siege of Venice, then a whole host of deadly conflicts would have never happened, and humanity would probably be politically unified right now. An explanation as to why that's a good thing is not necessary, obviously.
Also, inb4 "your parents conceiving you."
please don't
thanks