I believe sugar may be the hardest addiction to break because of social norms.
With heroin, crack cocaine, and cigarettes, most people in society will help you quit and support you.
Quitting alcohol is gaining more support because a larger percentage of people are joining that club.
Sugar takes between 2 and 4 months to break the addiction, and during that time frame you will be prodded to eat something loaded with sugar on a daily basis. Even from people that love you.