The human brain is regarded by many as the most complex "thing" in the entire universe. Whether you consider it a machine, an organ, the seat of the mind, or a sophisticated computer, the secret to its vast complexity lies in the humongous number of brain cells (aka, neurons) combined with their amazing capacity to connect, process, and send information to each other. Therefore, the ability to transmit information between neurons is the absolute key to a healthy brain that performs properly. Who's in charge of such a delicate yet fundamental task? Neurotransmitters, of course--the chemicals that deliver the messages from one neuron to the next! There are several classes of neurotransmitters, each performing very specific functions.
Dopamine--one of many known neurotransmitters--is intimately involved in important aspects of brain function, which makes it a key player in drug use and addiction. Dopamine is present in regions of the brain that control movement, emotion, thought, motivation, and feelings of pleasure. In fact, when you have a nice meal or listen to your favorite music, dopamine signals your brain that something important has happened that needs to be remembered, prompting you to repeat the behavior. Drugs can cause a much more powerful release of dopamine so that such natural pleasures can no longer compete because they cannot produce the same "high" that people remember and seek to repeat. Eventually, people become focused on obtaining and using drugs at every opportunity--although recreating the initial high becomes nearly impossible. Dopamine is also involved in other brain disorders that have nothing to do with drug use: the tremors and other symptoms that characterize Parkinson's disease, for example, are the direct result of the loss of neurons that make dopamine.
Addiction is a disease characterized by intense drug craving, drug seeking, and drug use that persist even in the face of extremely negative consequences, such as losing your job, doing poorly in school, getting arrested, or getting sick. Addiction develops because of changes to the brain caused by drug use. Initially, all drugs of abuse, either directly or indirectly, increase the activity of the chemical dopamine in the brain's reward centers, which is what makes people feel good. However, with continued drug use and excessive activation of dopamine neurons, the brain starts to adapt to the good feeling, so more of the drug is needed to achieve it. This causes people to become dependent on the drug, to feel bad when it is not in their system, and to seek and take the drug compulsively--without even thinking about it. Another way that drugs change the brain is to affect the ability to make decisions, such as judging what's important, what's healthy, and what's dangerous. The compulsive seeking and using of drugs even in the face of potentially devastating consequences is the essence of addiction.
That said, drug addiction is a treatable disease—with medications (in some cases) and behavioral or psychosocial therapies. Treatment should focus on a person's individual needs, since many people who are addicted to drugs also have other serious problems, including other mental illnesses. Drug addiction is often chronic (long-term), with relapses possible even after long periods of abstinence. But relapse doesn't mean failure—just that treatment needs to be reinstated or adjusted to ensure long-term recovery.
To learn more, check out "The Science of Addiction" at http://www.nida.nih.gov/scienceofaddiction/.
Some people are more likely to become addicted than others. The fact is that addiction is a very complex disease, which means that the overall risk of becoming addicted—once you start using drugs—is made up of a huge number of contributing risk factors at both the biological and environmental levels. These include the genes you inherit, the conditions during your prenatal development, the experiences of your early childhood, the parenting style at home, the nurturing quality of your school and neighborhood, the level of stress you are exposed to, and any vulnerability to or co-occurrence of other mental disorders you may have; all these just to name a few! This high level of complexity explains why it is so difficult to predict who will become addicted and who won't. This difficulty notwithstanding, there are well-known risk factors that help us to identify those who are most vulnerable and focus our prevention efforts. For example, addictions tend to run in families: if your parents smoke, you're more than twice as likely to smoke as someone whose parents do not smoke. Also, if somebody suffers from certain mental illnesses they are also more likely to use certain drugs and become addicted.
This question calls for a rather complicated answer, so it may be helpful to reframe it in terms of a more familiar subject instead of answering it right from the get-go. So let's consider, first, the similar question of: what food is the most fattening? It is easy to see that the answer should be: "it depends." The fact is that some people can eat bread and pasta till they drop, without gaining an ounce of weight, while others on strict diets can't even look at a plate of lasagna without putting on a couple of extra pounds. The huge differences in how people assimilate various foods hinges partly—of course—on genetics. Indeed, some lucky people have such efficient metabolisms that they can burn calories as soon as they are ingested, and others, less lucky, don't seem to find a way to get rid of them. Age is another important factor: dietary fats have very different fates if consumed by adults or by children. Finally, lifestyle and the life choices we make every single minute, like walking or driving, taking the elevator or the stairs, TV watching or working out, will also have a tremendous impact on how "fattening the foods we eat are."
For very similar reasons, scientists are reluctant to rank drugs in terms of their addictiveness. There are just too many moving parts (variables) in the equation that would need to be considered: genetic (e.g., drug metabolism), environmental (e.g., drug availability) and developmental (e.g., age), all of which play important and changing roles in determining to what extent (in fact, even whether) use of a particular drug will lead to addiction.
The fact that it is impossible or at least ill-advised to rank drugs according to their addictiveness value does not mean that we cannot identify factors that contribute to or influence the addictiveness of a particular drug. We can offer a nice example from the world of stimulants, where different routes of administration (i.e., how you take it) can have a profound impact on the ultimate risk of harm and addiction. Smoking cocaine (crack) delivers large quantities of the drug to the lungs, producing effects comparable to intravenous injection. These effects are felt almost immediately, are very intense, but do not last long. The high from smoking cocaine may last from 5 to 10 minutes. The high from snorting cocaine, on the other hand, comes on slower and can last for 15 to 20 minutes. There is evidence that suggests that people who smoke or inject cocaine may be at greater risk of causing harm to themselves—including becoming addicted—than those who snort the substance, because the drug reaches its brain targets so much faster. And route of administration is but one of many factors to consider just in relation to the drug itself--the dosage, combination with other drugs, setting in which a drug is taken, and pattern of drug taking are other variables to consider along with those noted above related to the individual and his/her environment.