Alexander the Great conquered the majority of the known world during his 12-year reign as king of the Macedonian Empire. His abrupt illness and death in 323 BC prevented him from bringing the entire world under submission, as he desired, but he still managed to conquer more land in just over a decade than anyone in history. Thus he is remembered as history's greatest conquerer.
It is difficult to know whether he would have been as successful in his future conquests if he had not met such an untimely death. It is certainly difficult to imagine any nation having sufficient military abilities to defeat him (neither Rome nor Carthage had become the stout military powers they would later be known as). However, history is full of examples of conquerers who pressed their luck one too many times, thus tarnishing their legacy. One need only look to Napoleon's failed invasion of Russia to recognize this fact.