Genghis Khan defeated highly advance armies with his unique war strategy and war machine.
Khalid bin Waleed defeated stronger armies with divine power.
Not that i underestimate the power of faith, however that is not all that is required to be a great warrior/commander.
Khalid bin Waleed had men under him, men who died like any other.
It was his great courage, steely determination, sheer brilliance and unmatched skill which made him victorious over many armies and top commanders which Persia and Rome threw at him.
You would do well to read in depth about his strategic vision and tactical brilliance which ranks right on top along with the likes of Alexander and Genghis Khan.
However Khalid also had another quality, he was the best duelist of his time and killed scores of enemy champions in one on one encounters which were a tradition in those days. In armed or unarmed combat, Khalid's great strength, supreme confidence, speed and lightning reflexes made him something of a living legend...a legend which only grew as he fought the best armies and champions in the world.
There was not a single Arab, Roman or Persian champion who took on Khalid and was not defeated. This is something most extraordinary for a commander in chief, which to me makes him the complete Warrior.
With his combined traits of military leadership, courage and fighting skills...he was a complete and peerless warrior/commander.
Khalid never lost a battle against the Roman and Persian empires. He was undefeated against two of the super powers of his time and this includes epic battles where the enemy had nearly a 100,000 Troops with Muslims at a disadvantage of 1:3 combatant ratio.
Another very interesting fact about him is that as a non Muslim he was the one responsible for inflicting the only reverse on Muslims when they were commanded by the Prophet Muhammad (PBUH).
Khalid was the commander of cavalry and lingered on waiting for an opening even though the infantry of his Army had already started fleeing. The Muslim archers thought victory had been secured and left their post on a strategic hill against the instructions of the Prophet Muhammad (PBUH).
Khalid immediately calculated his chances and flanked the Muslims and nearly routed the Muslim Army which had to retreat to the hills for safety.
This was a key trait of Khalid, he really never ever gave up and was always precise in his calculation and timing with a counter attack...even in seemingly hopeless situations, he never lost tactical clarity or strategic vision and this inspired his subordinates so much that it became common knowledge amongst all Muslim soldiers that if Khalid was in command, they would never lose any battle.