Whoa, Oxhead, you've come up with many of the great ones (I also adored the sense of humor shown by sending his stingy old tutor all that scent). But how about this?

Alexander's greatest achievement was in positing that a fusion was possible between the Greek World and the Persian World, that a blend of the two cultures would create a strong rule for Asia that would bring western civilization to it without destroying eastern civilization.

Since not ONE of his generals had the guts to even attempt anything like this, and in fact, no one else did until - perhaps - you could argue the Romans co-opted the idea for their Empire, might Alexander, if he had lived, seen a change to a Graeco-Persian culture that might have then changed world history?