Addiction in and of itself is neither legal nor illegal--any more than any other neurotic behavior.
I'd say the most difficult addictions to overcome are the ones that don't have any serious spotlight on them (like narcotics anonymous, alcoholics anonymous, etc.).
Pornography addiction is a difficult one to beat because the "triggers" are EVERYWHERE (movies, tv, radio, print ads). Sexual gratification in the male is probably the strongest physiological impulse in the human body--higher even than hunger (up to true starvation when the body starts to shut down). When that impulse gets tied to some addiction then it can be almost impossible to control. Impulse control is not quite the same as addiction though; so just because someone looks at pornography doesn't mean they are addicted any more than someone who sometimes takes a pain killer on a whim for fun. It's when it starts to control your life that you should be worried.
So, there may well be support groups out there for people addicted to pornography ... but what about deviant pornography? What about the stigma associated with that kind of pornography (snuff, rape et al)? Those are the kinds of people who can't get help at all because if they do try to get help then at best they are shunned and worst they are thrown in jail. I'm not talking about the PRODUCERS of those kinds of porn ... they do need to be punished by the law AS WELL AS receive psychological help. I'm talking about the "so-called" passive types -- who just need help getting over the addiction and impulsive behavior.
I'd say the next most difficult kind of addiction to overcome would be LEGALLY obtained prescription painkillers. I had back surgery a few years ago. I know exactly what to say and how to act to get a prescription for pretty much any kind of painkiller I desire. However, I don't do it. But once you know how to act (and a good doctor can tell if you've never experienced a ruptured disk) you can waltz in there and proclaim, "My back hurts!" and they'll write a prescription for 20 Lortab without a bat of an eye; especially if there is already a history of back pain.
After porn and painkillers, I'd say the next addiction would be pancakes or waffles.
(Actually, there is some truth to that ... food addictions are notoriously difficult to beat.)