I'd say the period is too long to pick one. Sure, the battles of the Islamic conquest (and its limits) were decisive, and by virtue of being ancient affected all the others... but by 1500 that hardly mattered as much as more recent battles defining the rise and fall of the Italian merchant republics and the subsequent rise of the Atlantic empires (which, no offense to the Ottomans, Mughals or Chinese, were ridiculously important by knitting a whole New World together with Eurasiafrica).
But, looking at trends, I'd be tempted to go with a battle that defined both the limits of Islam, the subsequent few centuries of Merchant Republics, knocked on into the Roman-Turkish conflicts, and go with the capture of Jerusalem in the first crusade.
While the Crusader Kingdoms did not truly last, the success of the first crusade in taking Jerusalem all but guaranteed follow-up crusades, including the 4th that knocked over the Byzantines quite hard. In addition it hacked out a niche for the Italian merchants in the Levant, and led to no end of their meddling (and acquisition of riches to fund the Atlantic expeditions). Finally, it also upped the ante of Christian-Muslim hostility, which undoubtedly added to the Portuguese desire to bypass the Muslims during their expeditions along the African coast.
Of course it means the battles that inspired the first crusade also count, as do the followups, so it gets messy.