Tough question--it's easier to ask which is the least dangerous, because the answer then is marijuana, and by a huge margin.
"Least dangerous" doesn't mean "completely safe". All drugs have risks and the best choice is not to use any of them. And young people absolutely should not be doing any drugs, because the dangers are magnified if your body and mind are still developing. Unfortunately, current policy fails miserably in this regard, because each of these three drugs will be tried by half or more of our youngsters before they graduate from high school.
For adults, though, the numbers are clear. Tobacco kills about 440,000 people annually in the US, and alcohol something close to 100,000, according to the CDC and many other sources, while the number of marijuana deaths is zero--or at least so close to zero that it can't be detected in epidemiological studies. Tobacco and alcohol each cause a long list of serious diseases--cardio-vascular, lung, and liver ailments, and many kinds of cancer. Marijuana causes bronchitis and excess phlegm production but not much else.
Two scary facts: 1) the Campaign for Tobacco-free Kids says that more than five million US kids will die from tobacco. The deaths will occur later, of course, but 90 percent of tobacco users acquire the habit before age 18. 2) Alcohol induces violent behavior in some of its users, resulting in assaults, murders, and traffic accidents (while marijuana has the opposite effect).
One reason "pot" is less dangerous is that it's much less addictive--although drug experts prefer the term "dependence". They estimate that 32 percent of tobacco users will become dependent--higher than any other drug, including heroin and cocaine--while the figures are 15 percent for alcohol and nine percent for marijuana.
Marijuana's risks are mainly mental. It certainly impairs short-term memory, probably causes some cases of schizophrenia, and inexperienced users may experience a feeling of paranoia that can cause an emergency room visit, but usually without serious consequences.
It also impairs driving ability, although much less than alcohol. The biggest study (in the December, 2005 British Medical Journal) found that drivers with a high blood level of THC (the main active ingredient in marijuana) were about three times as likely as a sober driver to have a fatal accident. But it found about the same increase in risk in drivers with a blood alcohol content (BAC) of 0.05 or less--that's just one or two drinks, depending on your body size. Really loaded drivers--with BAC above 0.20--had a risk 40 times higher than sober ones.
Unlike alcohol and tobacco, marijuana also has well-established medical uses, especially for controlling chemotherapy-induced nausea, boosting appetite in AIDS patients, and relieving neuropathic pain--without the dangers of opioid painkillers. And it may inhibit tumor growth, but this is not well-established. That's why 17 US states and DC have approved it for medical use, as have Canada, Israel, and the Netherlands.
The tragedy is that we knew most of this in 1972, when the US National Commission on Marijuana unanimously recommended that it be decriminalized (even though most of its members were appointed by President Nixon), and Consumers Union, in a major report titled "Licit and Illicit Drugs", recommended that it be regulated like alcohol. Since then more than 20 million people have been arrested for marijuana "crimes"--wasting scarce law-enforcement resources--and more than 10 million have died from alcohol or tobacco. It's time for a change.