Without the Normanization of England, it never would have been the great power it was destined to become.



Chaldiran [...]Selim the Grim [...] janissaries[...] Ottomans
Surely you mean Çaldıran, Yavuz Sultan Selim, yeniçeri and Osmanlı. Or do Turks not warrant native names?
Anyway:
Las Navas de Tolosa, 1212
The Reconquista was not yet a foregone conclusion. Half of Iberia was still Islamic, and twenty years earlier, Yaqub al-Mansur had inflicted upon the Christians their greatest defeat since Guadalete, and only luck saved Toledo. In 1211, al-Mansur's son was back, with an even larger army, to have another go. The Crusader force gathered to oppose him included contingents from every Christian power in Iberia, and significant trans-Pyreneean components. The Crusaders caught the Almohads by surprise and slaughtered them nearly to a man, with the Emir escaping by the skin of his teeth. The Almohads entered terminal decline, and the close duel between Muslim and Christian Iberia became a rout, as all Andalusia save Grenada fell to the Great Reconquest in the next forty years.
Nancy, 1477
In 1475, Burgundy was probably the most powerful state in Europe. The conquest of Lorraine and Alsace, to which Charles then turned his attentions, would have united the two halves of Burgundy, secured the entire length of the Rhine, assured Burgundy's place as the dominant power in both France and the HRE, and probably brought a crown. Of course, Lorraine allied with the Swiss. Charles and his splendid army met their end, as so many others, at the end of a Swiss pike, and instead Burgundy rapidly disintegrated. The Burgundian collapse completely altered the future shape of Europe, and the ramifications were not fully realized for at least another two hundred years.
Edington, 878
The winter attack by Guthrum's Vikings caught Alfred by surprise. He was nearly captured at Chippenham and had to run for it with a handful of followers. For somewhat more than three months, Alfred hid out in the Somerset Levels while the Danes had the possession of country. When spring arrived, Alfred summoned the militias of the country and crushed Guthrum. The victory saved Wessex and virtually established England; the consequences of defeat scarcely bear thinking about.
Lechfield, 955
Not so much for the battle itself. The Magyar threat had been receding for some time. The Lech campaign was something of a last gasp by the Magyar leadership, and if memory serves they'd gotten their arse kicked by the Bavarians a year or two earlier. What Lechfield did was firmly establish Otto as the de facto Emperor; Widukind of Corvey of course credited it entirely with his elevation. Had the battle not occurred, or had Otto lost, it seems likely that he would never have received the Imperial title. It need scarcely be said that without the Empire, and the vague oecumenical claims that came with it, the whole history of Germany and Europe would have been very different.
Lincoln, 1217
Bit of a wild card, and not really sure about it - Dover holding out might be more important. Anyway, Prince Louis of France, the future Louis VII, invaded England in 1216 at the invitation of the barons rebelling against King John - the Magna Carta having failed to quiet the situation. He quickly defeated the opposition and for a few months was the de facto King of England. John's death undermined the rebellion, and barons began returning to the young Henry III. Henry's regent, incidentally, was William Marshal, whose life story is one of the more interesting of the middle ages. Louis' forces were defeated by Marshal's at Lincoln, and the Frenchman was forced to abandon England.
Honorable Mentions: Pavia, Brunanburh, Diu, the Downs, Bosworth Field.
Plus Catalaunian Fields, Soissons, Tours, Lepanto, Vienna and Trafalgar, of course.
People keep mentioning the Battle of Tours as if it were somehow important. This is not the case. The Umayyad forces met there were a raiding party that was out to plunder parts of southern France and steal what they could, then hightail it back to Al-Andalus. They never had any intention of invading France or the rest of Europe; the first Muslims to do this were the Ottomans, nearly a millenia later.
I agree in broad terms, but there are a couple of problems with this view. First, pretty much the same thing could be said about Tariq's initial expedition to Hispania. It was only intended to scope things out, really, but when it far exceeded its initial goals the Arabs took advantage. If Charles Martel loses at Tours, there is no other effective power in Francia, and no one to coordinate resistance. Do you really think the Arabs wouldn't have tried to take advantage? Also, the claim that it was a one-off raiding party is disingenuous in the extreme. After consolidating their rule in Hispania, the Arabs mounted a concerted effort to extend their rule north of the Pyrenees, starting with the conquest of the rump Visigothic state in 718-19, and continuing through the attacks on Odo of Aquitaine that forced him to run to Martel, and the conquest of Provence. Five years after Tours, the Umayyads were in Avignon. At the very least, Tours kept France's Mediterranean coastline out of Muslim hands.