If I had to choose from your list, I would pick smoking, which is the single most preventable cause of major illness. In addition to the risk of lung cancer and emphysema, tobacco injures many systems of the body and raises risk of bladder cancer, cancers of the head and neck, and leukemia. It is especially dangerous to use tobacco if you are diabetic or are taking birth control pills or if you have any heart or circulatory problems, respiratory disease, urinary or prostate trouble, digestive disorders, a family history of cancer or increased risk of cancer for any reason, high blood pressure, seizure disorder or a family history of coronary heart disease.
Your list omits mention of an unhealthy diet. Eating refined, processed and manufactured foods is a major threat to health in the U.S. Americans are eating too much of the wrong kinds of fat and carbohydrates and are experiencing an epidemic of obesity that in turn increases risks of diabetes and heart disease and can cause or worsen many other health problems.
Not getting enough exercise also ranks high as a poor health habit. Our bodies evolved in very demanding environments and are meant to be used. If they are not used, they deteriorate faster than they should. Many of the illnesses that plague our society result from underuse of bodies. The prevalence of heart and artery disease correlates as much with lack of aerobic exercise as it does with unhealthy diet. Insufficient aerobic activity also predisposes us to musculoskeletal disorders, gastrointestinal problems, nervous and emotional illnesses and a long list of other ailments.
Stress is unavoidable in modern life and has physical and emotional consequences. The trick is to learn how to handle stress so that it doesn't take a toll on your health.
As far as the other habits you mention are concerned, I hesitate to rank them. Perhaps the most striking link between lack of sleep and disease comes from studies showing that the less people sleep, the more likely they are to become obese and develop all of the diseases associated with that.
If you drink alcohol to excess, you know it, and you also know that it presents huge risks to both physical and psychological health. Similarly, if you are addicted to illicit (or prescribed) stimulant or depressant drugs, I don't have to tell you of the dangerous risks you're taking. Not flossing? You could develop gum disease, which can lead to big dental bills, tooth loss and acceleration of atherosclerosis, the arterial disease that leads to heart attacks and strokes.
Don't worry, be happy...and don't smoke.
Andrew Weil, M.D.