imho RedRalphWiggum raises a good point. Unless you pick a constraint to deal with the remoteness of long-deceased actors, then there is little point in what-if'ing human history in comparison to more recent ones.
So i say if comparisons are to be made, the 'impact' should be one century out after the divergence, and no 'total' events.
But 'biggest change whose differences would be observable after 100 years, assuming there is still a humanity left to comment on it'.
e.g. no dinos with opposable thumbs making nuclear weapons, and no Romans colonizing Mars 500 years after they worked out a steam engine.

By this rule we can answer some of the following, and discount some of the others:
i'll tackle the last two. (1) The world would end. (2.) For that you would have to go back to the original events and deal with a couple thousand years of stupid propaganda pretending six million Persians invaded and three hundred very muscled, very oiled men in very tight codpieces Saved The World From Orcs And Other Eastern Monsters. [When in truth it was not six million, it was not 300, and plenty of Greeks got along better under Persian rule than they did at the hand of their fellow Greeks.]
So i say if comparisons are to be made, the 'impact' should be one century out after the divergence, and no 'total' events.
But 'biggest change whose differences would be observable after 100 years, assuming there is still a humanity left to comment on it'.
e.g. no dinos with opposable thumbs making nuclear weapons, and no Romans colonizing Mars 500 years after they worked out a steam engine.
By this rule we can answer some of the following, and discount some of the others:
What if Tang Dynasty China never happened and the use of paper never spread beyond the Chinese bureaucracy?
What if Genghis Khan died of a cold as a child?
What if Napoleon conquered Europe.
What if Egypt went the way of China and never lost dominance of their region?
What if Romans never invaded the British isles.
What if Zheng He's fleets were never scuppered and the Wall of China never rebuilt?
What if Buddhism never spread to China?
What if Christianity never spread to the west?
What if Caesar wasn't stabbed to death?
What if the Macedonians weren't such a pussies and just did what Alex told them and made it all the way to China?
What if Alex never went to the east and stayed in his already very large territory and actually did something with it?
What if Russians smiled more often?
What if Hollywood wasn't such a racist industry and didn't portray Xerxes as a 9 feet tall black African who wore golden undies?
i'll tackle the last two. (1) The world would end. (2.) For that you would have to go back to the original events and deal with a couple thousand years of stupid propaganda pretending six million Persians invaded and three hundred very muscled, very oiled men in very tight codpieces Saved The World From Orcs And Other Eastern Monsters. [When in truth it was not six million, it was not 300, and plenty of Greeks got along better under Persian rule than they did at the hand of their fellow Greeks.]