That's a fairly easy question to answer. If I could rid the world of one thing and only one thing I'd pick violence. The world would be a much better place without it. People would be more compassionate towards one another. Hatred would probably cease to exist as well without violence. Maybe video games would be a little less exciting and there wouldn't be as many action movies because if violence didn't actually exist it wouldn't get depicted quite so often. Overall I think the world would be a much better place. There would be more good relationships without domestic violence. There would be less psychologically damaged people without violent trauma witnessed or experienced in childhood. There would be no war and none of war's devastating effects.
Of course my second pick would be poverty. No more people starving, living on the streets, exposed to the elements. Without poverty third world countries would have better medical facilities to treat ailments so there wouldn't be as much disease. It is also likely theft would be fairly rare without poverty. The human condition would vastly improve if we eliminated poverty.
If I could rid the world of a substance but not a concept I would probably pick Meth. I have seen the destructive effects of the drug which seems to be the drug of choice in my small town. I understand how it enhances the pleasure center of the brain but it also causes loss of sleep, tooth decay, disorganized and erratic behavior and many more effects I have observed by watching friends. It also seems to be highly addictive, probably because pleasurable activities without the drug seem dull in comparison. Although I'm sure if it were eradicated something else would pop up in its place. I would expect the effects of it to be disastrous even if all I knew about it were the ingredients. It's basically made of household cleaners which are poisonous. In fact, making it is so dangerous that one wrong move can result in disaster (don't judge but some of this I learned from the TV show Breaking Bad).
I am not recanting my earlier statement that I believe drugs should be decriminalized. I think doing so would help people to get treatment earlier in their addiction and wipe out the drug cartels which are so prevalent nowadays. I'm just saying that if I could make it so Meth never existed, I would.