I think it is a close call.
Drugs are often more severe, because they don't only affect the individual, but cause a lot of crimes.
Alcohol, because it is legal (and yes, I enjoy my Bier once or twice a year and maybe a glass of good wine or Whiskey once every six months or so), is far easier to get and widely spread.
Any addiction can have a bad impact on those involved. I personaly had a hard time slowing down on coffee. I didn't know that it was addictive until I had to go to a two-week training course and must have been worse than when my Ex tried to quit smoking.
Smoking doesn't only get the smoker sick, but those living around them.
Alcohol can destroy families, carriers, lives without the individual necessarily realizing it. And it takes a strong individual to walk away from it. I have a few friends that are recovered alcoholics and they turned out to become much stronger people I learned to respect very much.
Drugs are a horrible thing. If you ever see somebody you love waste away... And the people that get missused, hurt or killed in the drug trade...
Gambling can wipe out entire families and leave them homeless.
Food addiction can leave its victims with severe health issues and emotional scars; especially through the harassment suffered by others.
I spend a lot of time watching my children, praying that they will never fall victim to it!