Here's my top ten. I look at the question in a very utilitarian way. This list may be subject to change.
1. Stanislav Petrov deciding not to respond to a false alarm of US attack on the USSR. Thank you so much. Honestly, I don't greatly believe in the Great Man interpretation of history, but he's definitely an exception...
2. Any country that decided to abolish slavery, and end the slave trade. Especially slavery in mines, sugar plantations, etc. I mean, I know there were lighter forms of slavery and even things like wage slavery, but limiting outright slavery, with all the cruelties it involved, was a start.
3. The creation of the vaccine and discovery of penicillin. I know they're not the same, and didn't even happen at the same time, but I'm going to combine them anyways. Life is good, right? Saved quite a few parents a lot of grief as well...
4. Mahatma Gandhi's decision to use nonviolent resistance against British rule. I'm going to bet that it saved quite a few lives, and led to other nonviolent resistance movements like the Civil Rights movement.
5. Augustus establishing the principate, setting the stage for the Pax Romana
6. Liu Bang’s decision to fight the feudal fragmentation endemic in China at the time and centralize it into the Han dynasty.
7. The establishment of a democracy, with the beginnings of basic human and citizen rights and of wider representation in the constitution of the US. Though it wasn’t perfect by a long shot, it set the stage for a lot of freedoms and benefits we all have today.
8. All edicts of toleration.
9. The US supporting the allies in WWII, so that the Nazi Regime would not be a world superpower.
10. Glasnost and Perestroika. Helped end the Cold War, finally.