I would say cigarettes. Worst stuff to ever be invented. Who in their right mind thought it would be a good idea to create something that you inhaled? The first puff making you choke and cough surely would be an indication that it might not be a good idea. But to continue on with it? Of course by the time it was discovered to be dangerous and cancer causing, most people are too hooked to quit. What's worse is NOW, in this decade, when we are fully aware of what happens when you smoke and kids are still willing to pick it up and do it. I don't get it.
(In all honesty, I was going to say something else (not political, I assure you), but I knew it would draw a lot of flack and most likely lose some of my friends here, so...)