He said that Alexander, Hannibal, Caesar, Gustavus, Eugene, Turenne, and Frederick were the greatest military minds in history, what criteria was he going by to assess their careers thusly? And what do military historians have to make of his list, are most in agreement that they are the greatest generals in history, or was he off the mark? Thanks!
For Napoleon, the most important thing for a man to do is to read histories. Napoleon, despite what we might think of him as a leader of men, general, or as some think as a tyrant, he was a huge bookworm. As such, he would read mainly histories with some inclusions of poems and novels, but he focused on history.
His youth was cold and lonely during his education in France. While he never felt included by the native French speaking cadets (whom made fun of his Italian accented French), he found friends in books and histories, reading everything he can and spending his allowance not on food or clothes (often he was sallow and gaunt with clothes not laundered for weeks) to spend money on books. He would even go so far as to translate Caesar's Civil War into French.
His obsession would continue well into his adult life, every campaign, he would order numerous books on the history and geography of the region he was bound to enter. Every spare moment he would spend reading, even tossing out books from his carriage if they didn't please him.
However, what is most telling is a small episode in his Egyptian campaign. His men were resting for a short time and he comes across his commander of guard cavalry, Jean-Baptiste Bessiers reading a romantic novel (as a note, romantic novels during this time didn't mean ones based in romantic love but rather in the romantic genre of literature). Napoleon grabs the novel and sees it, tossing it behind him in the sand and commanding his general to read something more appropriate for men as only women read novels.
All of this contributes to an important notion, Napoleon was obsessed with reading histories. There is no real assessment of how he came to it and no true historian worth their salt would argue X was the best general of all time. Further they all are of their time and used their own weapons and tactics that suited the armies of the time. Frederick might have been an adapt commander of a Greek army but it would be unknown how he would respond to Roman flexibility of the manipular or Marian legions.
Follow up question: Why did he omit Mongol generals like Subotai?