My pick for best is Alexander, then i would have to pick Hannibal then Ceaser ( I dont know anything about the other guy) I dont know why some guys understimate the persians. They held one of the greatest empires in history and Alex was able to topple them in such a short time. Their power was in speed and numbers (its what they needed to conquer such vast territory). They had great generals but not one nearly as good Alex himself. Yes, he did have a more disciplined army equipped with superior armor and incredible moral (due to Alex and the phalanx formation they fought in which let them feel secure atleast mentally) but the persians with there numbers,speed, and good leaders would give anyone a hard time, even hannibal and ceaser. Alex fought head on and did not need to use ambushes to defeat armies atleast 3-10 times bigger than his. Yes, a good amount of them were slaves but remember any man can kill another man, heck a women or a kid could kill a man. Slaves were not wimps. The other men were proffesional soldiers. THe persians had hundreds of Calvary. Even more archers and just a plain stupid amount of infantry soldiers. Yes, ceaser had to face many gauls but not nearly as much and their generals were nothing compared to ceaser, hannibal, Alex or or most other generals. When Alex reached India he had to face an army probably as big or almost as big as he had to face at gaugamela. Their army was was of the same type as the persians except for the fact that they used Indian elephants. Obviously India was a much tougher situation for Alex than much others and he came away with draw. I do not believe that hannibal would be able to fight the indians and come out with a victory (with the men he used to fight rome) mainly because strategy can only do so much and the Indians were famouse for having such vast and strong armies (though they never expanded like the persians for variouse reasons) In my opinion hannibal and ceaser could conquer persia but i dont think they could of done it as efficiently as alex could have, but we will never know will we?