The greatest triumph of the Ottomans, I would argue, was most likely their conquest of Constantinople/Istanbul in 1453. The corresponding factor there is how weakened the Byzantine Empire became from the earlier actions by their fellow Christians of the Latin West (Europe)! While there were many enemies of Byzantium in the wet, the most effective was Venice, a burgeoning commercial and military rival.
The singular and most significant blow struck by Venice in the 4th Crusade (1202-04) when in 2004, instead of attacking Islamic Egypt to free up the Holy Land, the Crusaders instead attack and sacked Constantinople, wrecking their economy and essentially dooming that polity to the eventual Ottoman destruction. A true irony of history but typical of national and financial rivalries.
Some historians argue that the sack of Constantinople by the Venetians marked the real end of the Roman Empire.