"Highest impact" in what form and for whom? A tribe in Amazonas might not even notice. Also, i.e. "nukes to everyone" might destroy our world, maybe better to ask for "best impact"?

https://www.quora.com/How-much-energy-would-it-take-to-crash-the-moon-into-the-Earth Probably crashing the moon into the earth by slowing it down.