I always liked Alexander the Great, for Great is the best description of his life. Macedon at the time was a tiny nation, but his father Phillip II put it on the map with his military conquests, and Alexander took the army he was given and went to the extreme.
Think about it. More than once, his army of roughly 40,000 soldiers defeated forces more than twice that size, through brilliant maneuvers and taking advantage of his surroundings. Some accounts say that in the battle of Issus the Persian emperor Darius III had over 100,000 men, and Alexander still routed his opponent and personally forced Darius to flee the field, after which he was captured and executed. He conquered the largest empire in existence and even took his men all the way into India. While his campaign stalled at that point, it was only because his forces were stretched so thin that he decided to turn around and consolidate his vast empire. Oh yeah, and he did all this as a man in his teens and twenties.
Judging a man to be great based only on military prowess and how many people he killed may be a bit grim, but he was the most successful general in history to this day, and his conquering of most of the known world was what created Hellenic culture. Alexander's strategy is still studied by modern commanders. His mark on the world is undeniable.