This is a highly debated question, and I wanted to see how this forum would respond, and more importantly, who they would chose.
One important note: Please support your choice with some piece of evidence.
Edit: For this topic, greatest means having some of the hardest odds, but still conquering a large area. This does not mean that someone who has faced hard odds but only conquered a small portion of land would count.
Replying to my own topic, I would say Chinggis Khan, more well known as Genghis Khan, is the greatest conqueror in history. Chinggis Khan was illiterate, and unlike other conquerors, did not come from a rich or well known background. After his father, Yesugei Khan, was killed when he was young, he, his siblings, and his mother were all abandoned and sustained themselves on small game. For a period of time, he was a slave, but he escaped. From this, Chinggis Khan created an empire from China to Persia. He started out with 100,000 soldiers and conquered millions.
When I was traveling through Central Asia and asked at many of the impressive archeological sites who had conquered and ruined them the answer was nearly always Gengis Khan. But Timur/Tamerlan conquered a huge but not long lasting empire too, like Gengis Khan with enormous brutality. And I have read that Alexander the Great killed ca 1% of the World population of his time in his conquest. Nevertheless we are all fascinated by these successful conquerors but should not forget the price millions of people had to pay for their ambitions.
While Ghengis Khan’s empire was certainly far larger than Alexander’s, I would vote for Alexander the Great. Why? Because I think what he did was absolutely insane. He set out with an army from a small backwater kingdom against the greatest empire in the world he knew. He then declared himself a god and went and defeated the Persian Empire, against all odds. He shouldn’t have succeeded by all logic but he still did.
Think the term greatest here does a lot of heavy lifting.
I imagine there’s various metrics for who would be the greatest conqueror of all time. Mentioning Genghis is the go to typically but what about Cyrus The Great? what he did with Achaemenids was near miraculous as well.
Mentions of Alexander also do belong in this conversation. Think a more direct pointing out of what associates the term greatest in most regards would help a lot with this question.