Surely Hannibal can't really be considered as the greatest general of all time because he was matched and deafeated by his own contemporary Scipio Africanus?

The Mongol Khans were excellent war leaders, but their conquests and empires were vastly linked to their lives. Whenever the Khan died nearly the entire army had to return home. This is why the Mongols never reached western Europe. Same as Alexander really, his empire was linked to his person and empires tied to personal powers are doomed to falter when their leader dies. Often a successor can emerge in such empires, but this can take time and the intermediary civil strife can put to waste much good work.

In short, many of the great generals discussed here failed to build lasting legacies because the greatness of their own kingdom was bound to their own mortality. In short, if you want to take all the glory of conquest and victory yourself, then the responsbility for its maintainance falls solely on your shoulders and if you cannot create a dynasty, all that will remain of your achievements will be legends and memmories.

The British Empire was able to remain so large for so long because by the time of its major expansion, military power was no longer in the hands of the King.