Alcohol.
Tobacco.
Cigarettes.
Nazism.
Communism.
Alcohol and tobacco will be near impossible to ban. I have nothing against getting rid of smoking, I find that it can be quite intrusive. Alcohol is fine in my eyes as long as you don't get drunk. Unfortunately, many people lack the self-control... 
As for Nazism and Communism, people can believe in these things by all means, but if, for instance, a neo-nazi were to attack somebody for racial reasons, then that's a no-go. You have the social liberty of belief, but no right to intrude on the liberties of others.
Unless, of course, you are limiting their liberties to prevent them harming the liberties of others. All hugely complicated. Just be a considerate person, basically.
Banned with an inordinately large hammer. 
