Originally Posted by
m1k3 
Is alcohol addictive in the same sense that heroin or nicotine are?
Yes, and having been addicted to both alcohol and nicotine, I can say that the mechanism of addiction is very similar. Granted, alcohol has a very different withdrawal syndrome, and in my case, lasted much longer than with nicotine, but otherwise, there is not much difference.
That said, some people seem to have a defense that protects them from addiction
to certain substances. I know people who can smoke cigarettes moderately, for example, whereas my body adapts to nicotine very quickly. I can't speak for heroin, but I do know that some people will say it is more addictive then alcohol, and vice versa.
Bottom line, though, the "most addictive substance" is the one you are addicted to, and the only substance that really matters is the one that does you in.