Alright, time I made my list, so here they are in no particular order.
1) Marathon (490 BC) -Platea (479 BC) -Salamis (480 BC) - each of these battles accomplished the same thing and it would be unfair to include one without the other. Had the Greeks lost any of these, the world would have turned out very, very differently.
2) Gaugamela (331 BC) - far more important imo than Issus, Granicus, or the Siege of Tyre. It was the last straw for the Persian Empire, and Alexander's forces were outnumbered and had a very stretched supply line.
3) Adrianople (378 CE) - this caused the collapse of the Roman Empire. I think that the Roman Empire and its collapse are probably the two biggest events that shaped Western European history.
4) Yarmouk - (636 CE) - had the Byzantines won this one, this little religion called Islam would be a very marginal thing at best, and the wars of the Calpihates would never have happened. No Battle of Tours, no Moors in Spain, nothing. in my opinion, this is more important than the siege of constantinople that would occur in the next decade(s?). This battle ties in closely with my next one.
5) Manzikert (1071) - this battle was the reason the Byzantine Empire collapsed and the Turks remained in control of the Muslim world. This is far, far more important than the Siege of Constantinople 400 years later.
6) Kulikovo (1380)/ Ugra River (1480) - I'm not sure which of these more properly fits the bill, but together they created the Russian Empire and overthrew the Golden Horde.
7) Siege of Vienna (1529/1683) - the first one I think is the most important one, though the second siege of vienna should not be discounted. Moreso than the naval victory at Lepanto, this land battle was the high water mark for the Ottoman Empire.
8) Spanish Armada (1588) - this propelled England from an average sized European state into a superpower. The destruction of the Spanish - at the time the most powerful Empire in europe - was more than significant, and its consequences allowed England to begin colonization.
9) Trafalgar (1805) - this is here because of the consequences Europe would have faced if the English had lost. Napoleon's continental system would have been enforced, England would have been marginalized, and who knows how long Europe would have lingered under Napoleon's control?
10) Operation Market Garden (Sept. 1944) - now hear me out. Of all the battles in World War 2, why pick this one? Think of post-war ramifications. In August 1944, the Wehrmacht was routed in France, in the worst of ways. Had the Allies succeeded and bridged the Rhine in autumn 1944, imagine the consequences! First, the Wehrmacht wouldn't have had time to re-group, and second the major industrial base of Germany - the Ruhr - would have been taken by years end. Imagine the huge leg-up this would have given the western powers in the post-war scenario, of British, French, and American soldiers taking Berlin in the spring rather than the Soviets, of an undivided post-war Germany. There would still have been an "iron curtain" but perhaps it would have been pushed back a bit farther.
Honorable mentions:
Agincourt (1415) - this (along with some earlier battles) was the end of cavalry as the dominant military force, as well as a significant English victory. But the military strategic shift Agincourt caused (as well as Crecy and few earlier battles with the Dutch) was very significant.
Cannae (216 BC) - now this one might seem odd, since Carthage didn't win the war, right? Well, I put this one up here because of the changes it caused in how Rome conducted warfare. After this, Rome fully mobilized its entire population and more importantly changed military strategies. Instead of big army vs big army battles, Rome adapted and went for many smaller - but still sizable - armies which would take strongpoints and cut supply routes and win over individual city states. It is this strategy that forced Hannibal to retreat to Carthage and forced the Battle of Zama, and it is this strategy that the Romans kept throughout their empire.
Carrhae (53 BC) - it marked the farthest the Romans would go in the Middle East and let this little guy known as Julius Ceasar to come and become Emperor and turn the Roman Republic into the Roman Empire.
Teutoberg Forest (9 CE) - the destruction of three legions by barbarians was a pretty momentous event, and it prevented Rome from expanding past the Rhine.
Poltava (1709) - this could have been the end for Peter the Great's Russia, and Sweden could have been the major power in northern europe for a long while more
Failed invasion of Japan (and Java) by Kublai Khan (late 1200s)
Failed invasion of Korea by Japan (1500s)
Saratoga (1777) - this was the decisive battle and turning point in the American revolution and the creation of the USA
Hastings (1066) - the ramifications of having Norman kings would drive England out of isolation and into European affairs
Tsushima Straights (1905) - The rise of the modern steel battleship as well as the Japanese nation
Verdun (1916) - this more than anything else broke the back of the German army in WW1. The failure to capture this series of French fortresses was of more consequence than Somme, Ypres, Marne, or any of the other major German offensives
And many, many more