Don't know if someone already mentioned it, but I suggest:
The Battle of the Plains of Abraham (Without it then things would've been vastly different in North America)
Seconded.
It was the most decisive battle in North American theater of Seven Years War, even more decisive than siege of Louisbourg I think...
And about Seven Years War, there is one decisive battle in Indian theater:
Battle of Plassey, 1757 AD, British vs French and Indians, which lead to British annexation of Bengal and ensuring the domination of British East India Company in India for the next 200 years...
And then there are some other battles:
Siege of Orleans, 1428 AD, English vs French, a turning point in Hundred Years War...
Siege of Constantinople, 1204 AD, Byzantines vs Crusaders and Venetians, permanently ended the Byzantines' role as the "shield" of Europe against the Turks...
Battle of Lechfeld, 955 AD, Germans vs Magyars, ended the Magyar raids in Europe and lead to the establishment of Holy Roman Empire...
Battle of Pharsalus, 48 BC, Caesar's forces vs Pompey's, starting the Caesar's domination and ended the Roman Republic...
Battle of Chaeronea, 338 BC, Macedonians vs Athenians and Thebans, marking the start of Macedonian domination over Greece...
(I nominated too much battles, didn't I?