I once spoke with someone who said the two worst inventions were the TV and the car. The TV for obvious reasons. The car because of the societal dependence on gasoline and corresponding decrease in regular exercise.
Dallas Mavs...whoops... Seriously though, I'd say cars. It takes up so much space, inefficient, and anti-social. Dependence on gas, expensive, and contributes to the laziness and obesity of Americans.
off the top of my head... land mines speed bumps mayonnaise cigarettes Motion Picture Association of America (MPAA) Recording Industry Association of America (RIAA)
the IRS and its tax code professional lawyers the pet rock fast food and its ad campaigns (I'm lovin' it...http://www.i-am-asian.com/) refined narcotics
Biological weapons - Nukes and chemicals are bad but you don't have to worry about nukes or chemicals reproducing or mutating on their own. The whole idea of bio weapons seems like such a bad idea because one mutation and you're in as much danger from using them as your enemies. Gasoline and other fossil fuels - At the start of the internal combustion engine age there were renewable bio-fuels available and if we hadn't adopted fossil fuels as a primary energy source we may have had far less problems with our environment and geo political situation. Cigarettes - They not only harm you but anyone else near you too. High fructose corn syrup - That stuff is in pretty much every processed food now. Drive-through windows - How much trouble is it to get out of your car to walk in and get your Big Mac? Not only are you stuffing yourself with calories but you're missing out on burning a few of those from walking and standing in line along with wasting gas and polluting the environment as your engine idles while they get you that extra packet of bbq sauce. Shopping scooters- I'm talking about the kind that you can check out at grocery stores to do your shopping. I understand that there are people who can't walk and that's why we have wheel chairs. That we need shopping scooters for the morbidly obese to get groceries just exacerbates the problems.