Easy, Napoleon is leauges ahead of Khan or Alexander.

The other 2 may have been more successful in terms of conquering but Napoleon far beyond in terms of skills and tactical genius. Although Alexander and Khan had good victories in Persia, it really doesn't compare to Napoleon's feats in Ulm, Marengo, Austerlitz, etc....
Heck even in terms of opposition Napoleon is ahead. Theres not a single commander that Alexander or Khan faced that could be considered as good as likes of Archduke Charles, Wellington, Nelson, etc...