I've been through daily-use heroin, benzo, and amphetamine withdrawal, and it is without a doubt benzo withdrawal. Two straight years of post acute Hell. Constant anxiety, daily panic attacks, extreme antisocial behavior, persistent physical tremors.
Not to mention I've since been able to use both heroin and amphetamines without going off the rails very shortly after I start. Psychological addiction is a big problem, benzos eliminate the anxiety issue I already had in the first place better than anything else I've tried.
Probably alcohol or benzos. I really feel for severe alcoholics because it's impossible to avoid alcohol (unless you move to Utah and become a Mormon I guess). Leaving where you used to get high and use is a great way to change, but it's impossible with alcohol because there's liquor stores everywhere and there's no negative connotations with drinking. Benzos are also tough because most people don't respect how difficult they are to quit and how insidious their use becomes in your psych, etc. Also, Doctors are fine prescribing them indefinitely so you might not try to get off em for 10+ years and then it's really tough.
Just roughly I wonder what drug would be toughest to quit including the paws of course weather they are short or long. When it comes to opioids they say it's fentanyl. IME it is the evil suboxone. Well it's less evil than getting od but I hated it it gave me more than a year of depression.