What single decision by one individual, in your opinion, saved the most lives in history?
My answer is when Soviet officer Stanislav Petrov prevented an all out nuclear disaster in 1983, making the decision to not launch a retaliatory nuclear strike on the United States when the Soviet missile detection system malfunctioned, giving the incorrect signal that a nuclear strike was inbound. His decision to veto the other officer's impulsive decisions likely saved not only millions in the United States but also in the Soviet Union. If nuclear missiles were launched at the U.S. at the time it's possible a full on nuclear exchange could've occured.
Edit: Turns out the Petrov story is false, or at least overblown/glorified here's u/joey-jo_jo-jr 's comment for context:
"The system was relatively new and known for throwing up false warnings, a similar issue had happened a few months earlier, therefore everyone knew that the supposed US launch was likely a system error. Petrov flagged it as such and his CO came down and confirmed this. Even if Petrov had flagged it as a genuine launch his CO would have flagged it as a system error. The duty officer at Soviet air defence in Moscow also called them and told them that their system was the only one showing a supposed US launch, no one else could confirm it.
Regardless the Soviets would not have launched a retalitory strike until they had visual satellite confirmation of a US launch, i.e. they could literally see missiles heading towards them"