That depends. To what purpose?
If I am trying to save the world, I'd remove the most destructive force in it: humans.
If I'm trying to save humanity, I'd remove the most destructive force in it: religion.
If I'm trying to save myself, I'd remove the most destructive force in me: the ego.
If I'm just removing something for selfish fun, I'd have to go with systems of oppression. That might automatically bring about my first suggestion, but at least with this phrasing, I've tried to leave some of us behind.
Or
Hatred in all of it's many forms is mankind's primary downfall and may someday be our destruction. Why do we hate? Sure we can dislike, distrust or rationalize why something is wrong but when we hate it becomes unreasonable and irrational. It becomes a self fueling action. Hate robs us of what it is to be human by robbing us of choice. We feel hatred we do not think hatred and you can not reason someone out of something they were not reasoned into.
Or
Abuse. Of any kind. Physical abuse, mental abuse, emotional abuse, domestic abuse, self abuse, alcohol abuse, drug abuse, child abuse, elderly abuse… any abuse. Get rid of abuse and I feel like the world would be a much better place.
May Jesus Christ bless each and every poster here. God Bless.