Trump winning 2016. He needs to loose, so he never attempts again.

The tiktaalik that though leaving the waters is a good idea, or the monke that thought walking up right is a good idea

Newton's third law.

The collision with Theia

A certain guy named ~~Davel~~ Putin, he's Stalin's aggression mixed with Beria's inconsideration at this point.

If I Thanos snapped myself, wouldn't that just delete half of me? Left or right? Top or bottom half?

I'll snap the neurons that cause you to post this question so that

[̸̨̛̙͖̝̣͚̫̼̟̉̾͐̓̀Ę̶̠̻̺̟̖̙̠̻̱̟̀̾̾̒̒̅̎͑͝R̷͓̯̘͇̪̥̭̳̙͛́͝͝R̷̛͍̟̆͆̓̆̐̌̃͠Ó̴̤̟̪̦̍͗͒R̸̡̰͐̃̚͝:̴̮͈̜̺̐̇ ̵̢̛̝͎̖̘̤͈͌̿́͌́̄͗͌Ṣ̷̼̅͌̈́̓̎͆͂̚͝p̶̳̼̹͌̑̍̈́̍̓͋̌͗͠a̶͓̜̦͇̖̩̫͗̊͒̽̈̇c̶̯̰̺̖͈͕̎̒̄͌̒̑̓̏̽̽͝ͅͅe̵̢̜̗̰̠͌͗̈́̐̃̄̄-̴̛͍͕͔̪̗͗̓̊͑̅̃͝T̵̡̡͉͚͉͓͑͐͜ͅị̶̧̯̘̦͖̞̥͇̦̥̓̃͆̃̕m̸̪̓͛͛͗̅̏͑̔͆͝ē̷͎̺̞̤̑ͅ-̵̧͙̠̖͓̣͙̥̟̫̽̉͐̒Ç̵̪̣̲͖̬̜͉̰̍̋̏͘ͅǫ̷̮̼̱̼̘̻̳͓̟͋̃̌̄͌͐̌͆̈́ͅn̶̛̲͖͔̈́̓̂̒̂̀͗̿͝͝t̴̻̼̺̤̦͑̄̔̋͑̿͘ͅì̶̲̲͈̩͇̮͓̱̭̠̈̓̔̉̅̅̅̕͝͠n̸̻̩̬̦͛̅̏̿́͐͋͂͜͠ů̴̳͉̫̹͍̦͎̠̘̿̆̀̔u̷̧̱̹̙̠̮͖̓͒̓͊̾͊͛͋m̸̥̣̤̿̅̏̃͌̂̀̌̓͛͝ͅ.̷̫͕͚͒̍̈́̃̏̚̕e̷̛̘̻͈̱̜̓͆̎͆̓̉̃x̸͉̥͓̬͓̑́̄͝e̵̳̻̘̠̺̽͊͆̾̋͗ ̴̡̢̢̼̖̫̱̗̼̻̮͛͋͆̈́̈́̕̕͝h̷͕̩̥͔̣̏̄̌̏͜ã̵̡̧͇͚̭͑͌̍ͅs̵̹͙̼̳̖̟͍̼͖̗̻̔͒̆ ̸͇͋́̌͋̿͝c̷͙̣͍̈́̈́̑͆͝r̵͓͕̯̼̝̤̫̬̭̊a̵̡̟̅̃̚a̷̰͙̥̼̰̍̇͂͗̈̅̄a̷̡̧̖̗͔̘̜͒͒͑̋̋̀̾͋͛̓̓a̵̧͍̭̮͒̂͐̍̌̒͐a̸̧̺͒̇̋͑̇̑͛̌̎͘͘a̶̩͒͘͝

I'd go back to the very beginning, the Prime Mover, and prevent all of it from happening in the first place.

We can propagate effects through time? Perfect. I'm going send a snap forward into the future. From now on, anyone that attempts to gather up the infinity stones gets instantly dusted.

I think the birth of Hitler is an easy choice. But I wouldn't be surprised to learn that eliminating some earlier event could both right another wrong and negate Hitler's existence.

Aside from that there's the existence of the internet, which I'm not sure has been an overall benefit to humankind. I think about this often.