... okay, I had to do this one - did this first bit above the edit without taking a look at your post, so I'm going in this truly blind. :P

two questions to start:

• a) what constitutes an 'event'? basically an immediate isolated flash-in-the-pan 24hr-or-less thing or the culmination of plans? (tried to go more for the former since they're more interesting.)
  
  
• b) most important by 'sheer number of lives it's affected at that time' or 'that multiplied by the ones that came after as well'? (second one being heavily weighted towards ancient events...? but then again due to world population explosion, the first is p weighted too... hm. again, went more for the former just because my awareness of modern history is _slightly_ better. :P )

aaand the list:

• first flea to human transmission of black death -> shaped politics of europe, both at that time + later
  
• my islamic history is a bit fuzzy; but that moment when the suuni and shiite families split? shaped a hell of a lot of countries, again, then and now.
  
• martin luther's thesis/usage of gutenburg printing press -> kickstarted information age & shaped major religions of today.
  
• declaration of independence signing -> prob. biggest thing that gave usa leverage to be a world empire? unsure of this one.
  
• assassination of ferdinand guy -> p obvious; ww1/ww2, politics.
  
• hiroshima & nagasaki -> bitchslap in everyone's faces that we're dealing with another dimension of war and/or true annihilation if we're not careful; started cold war and space race.
  
• tiamman square -> my chinese history ain't good either, but given how many folks are in china, seems like it's a flashpoint for censorship + political waves from that? it's either that or mao's inauguration due to the famines, idk.
  
• cuban missle crisis (specifically when the soviet sub captain ignored orders to launch nukes) -> the one 'negative' action of the bunch, meaning the 'what could have been' is more important - imo nukes raised the stakes on this kind of a question because you couldn't impact the number of people at the same time back in the old days.
  
• vaccine/penicilin discovered-> all the lives saved from that + changes to medical industry.
  
• torn between a few like the creation of israel as a country (again, religious/geopolitical) importance, british empire flashpoints, etc. i give up here. :P

now let's see what you wrote! :D
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post-read-edit: -does a fist-pump that we were so similar-! :D oh man this turned out even more interesting than I thought. Several further blurbs:

• Actually thought about the wheel/fire thing! but imo, that feels like it could've been an event that happened several times in isolation among various tribes, so there was no true *single* event that could've had quite the domino effect?
  
• Thought about Mongolia too ... maybe there was a specific event in Gengis Khan's life where the advantage for him turned? The hard bit in focusing on 'specific' events is that you've got these major players in history where it's difficult to figure out what day is most important out of all of their days, hm...
  
• Amused that we had Israel/American Revolution/WWI in common, plus a few similar generalities like Islamic history. Loved the specific ones that your friend had - kicking myself for missing the Yalta conference, durrr.
  
• darkly amused that nuclear events were a high priority on my list compared to others? not sure if that's because of my general focus on military lit or not...