As far as conquering Alexander

As for creating an empire Ghengis Khan. He looked to the future and even after his death the empire expanded.

A bit unfair to bring Alexander into this as he was'nt expecting an early death in his early 30's.

Would be quite interesting to see what happened if he lived to a ripe old age. But too likely as vast as an empire he created in such a short time, would fall apart upon his death. Just for the fact all the other kings would see it as a time to rebel.

Napolean is quite out of the question IMO as his empire did'nt even last his lifetime.