Well let's see, the candidates, Hernan de Cortes sacked Central America and may be responsible for the rise of Mocern Europe as riches poured into that continent through Spain. But although Cotres' conquests propped up the mighty Spanish Empire of the 16th century, his accomplishments are not on the top of my list.
Cecil Rhodes or Bill Gates? Rhodes represents an age of whimsical conquests as Europeans exploited dramatic advantages against the undeveloped world in a rare combination of will and ability to conquer. But no Imperial power could dominate the other.
Napoleon or Hitler? well they got off to brilliant conquering starts but were unable to consolidate the deal and they ended up embarasingly defeated.
No, I'd place Bismarck above either of them.
The Romans were perhaps the greatest collective conquerors, but I can;t pick one who tops my two remaining names.
Alexander the Great conquered the nearly the entire world he knew of and left behind a dramatically altered world. Yet he was not able to inspire a culture to hold together and dominate the majority of the world for long. His succcessors eyed each other jealously and tore into each other.
No, I am late to this answer. Temujin, who took on the title Universal Ruler or Genghis Khan, came the closest anyone has ever come to ruling the world. He conquered Asia and left behind a reasonably united empire with several succesors who accomplished what Napoleon and Hitler failed to do, conquering Russia.
The Mongol military might, pushed to greatness (if we are to admire the art of conquest) by Genghis Khan, was stopped by divine wind and a timely (for Europe) death of Genghis Khan's successor, Ogodei, at the cusp of conquering Europe.
The Europeans, like the later Japanese survivors of the great Mongol Armada under Kublai Khan, were left wondering why they were spared a military conquest. It was Obodei's death that revealed the weakness of the Mongol Empire, secession. Oh and as the Japanese learned, while they had the greatest mounted military units of their age, they were not experiences seamen. The Japanese remembered the Divine Wind that saved them forever in the name kamikaze (three parts vodka, one part triple sec, one part lime juice

), while the Europeans accepted there good fortune by forgetting that Poland once had the greatest army of knights of medieval times. (Anybody else getting a guilty flashback to Cable Guy?)
link 1link 2link 3link 4Genghis Khan's legacy marked the world for centuries. They included a long doninance of Central Asia, a Mongol Dynasty in China, two empire in India first under Timur the Lame, or Tamerlane and then under Akbar and Babar is the Moghul Empire.
And that defeat, too, was momentous, as the Christain Crusaders had aligned themselves with the Mogols and were subsequently driven out of the Middle East effectively ending the Crusades.
Persia, Iraq, Europe, Asia, Japan, when armies met, the Mongols eventually prevailed. Only in Africa was a Mongol force defeated. Then by an Egyptian force (Alexander's revenge?)