I agree that the answers are going to vary a lot on what we consider “most addictive” to mean. Let me throw a few more considerations in.
At first, I thought of the “you can die from the withdrawal” factor as well, and I was thinking of barbiturates. As mentioned, if someone who is seriously dependent is taken off barbs cold turkey, they are very likely to have seizures which will kill them. Acute alcoholism, as has been noted, results in DT’s during withdrawal which is very similar.
However, I don’t think this makes these the “most addictive” drugs - a large segment, probably a majority, of the population uses alcohol in moderation, or even to occasional excess without becoming alcoholics. Many people drank WAY more than moderately at some point in their lives, say during college, and required no special treatment to tone down that behavior to acceptable levels for a responsible citizen later on. Seconal and its brethren are drugs that a doctor may prescribe, and many patients have had such prescriptions without becoming dependent, though the dependency potential is high.
If you base things on the certainty of becoming addicted if you mess with the stuff, I think you have to rate opiates and nicotine very high. Although, morphine and other opiates are available for medical use, including heroin in some countries. I’ve heard it claimed that when opiates are used as painkillers, if the dosage is carefully regulated to simply balance the symptoms, dependency does not occur.
If we base our conjectures on the extremes of behavior in addicts, I think we wind up with methamphetamine and cocaine in their various forms, particularly smoked (crack and ice).
If we base it on the frequency of addiction in the population at large, nicotine and caffeine are far and away the winners. If all of the coffee disappeared from the planet, a huge number of us would have severe headaches tomorrow. I write this while getting my personal morning fix of “Peet’s”.