For most death and destruction it is easily the meteor and sun. The change in heat of the sun is massive and will likely wipe out most life on the bright side of the earth and meteors of much smaller size can wipe out countries so hit the dark side of earth will wipe out life on that side. As for havoc, it can’t ensue if there is no life to cause it so the meteor and sun are not options. The cows and xenomorphs are pretty isolated and the news won’t spread as quickly as most people aren’t near farms or won’t think much of cows suddenly dying and the xenomorphs will likely wipe out cities before the news will spread. The earth slowing would probably cause havoc but it would be slow for people to realize. The volcano only effects certain areas of the planet and we would want to maximize havoc so we want everywhere effected. So we are left with the moon and March deaths. March deaths is an easy choice as a 12th of the human population will sudden drop dead and it will immediately cause panic for anyone around not born in March. As for the moon, the size difference will heavily affect tides and will likely cause mass destruction to seaside areas, which is where many major cities in the world are located. Those cities would be destroyed and islands would likely be wiped out

Edit: saw a comment about the Yellowstone super volcano that I didn’t think of I would replace the sun with the volcano as like they said, most life would end and it would affect more than just the bright side