I don't think you can talk about "one" most important part. There are several things people have to do to stay healthy, and should do in order to keep health costs and doctor visits down.  Eat a healthy diet, get adequate exercise, don't smoke, drink only in moderation, keep exposure to chemicals, radiation, and toxins down, keep stress levels managable, and have good personal safety practices (wear seatbelts, practice safe sex, etc.)

Diet is certainly crucial to good health. But emotional health is as important as physical health, and high levels of stress can be a detriment to being healthy. Stress can be caused by emotional negativity, dysfunction in the family environment or workplace, high levels of noise on a frequent basis, a disruption in physical routine such as interrupted sleep patterns, or a lack of stability in one's life.

Other factors in personal stress levels can include financial problems, relationship difficulties, sexual problems, or anxiety about work, school or social situations. Reduction of stress plays a crucial role in improving the well being of patients with high blood pressure, heart disease, headaches and immune system disorders.

In addition, the physical environment plays a role in health. Living in an area with high levels of airborne pollution can be harmful to health, as can exposure to polluted water, noxious odors (such as those caused by paper mills or large farming concerns), loud noise or high levels of radiation, radon or cellular tower activity.

Only one of the most important issues, although some recent studies have suggested that in terms of weight loss, diet is more important than exercise.  Americans have grown more stationary over the decades, as we become more closely tied to technology (like typing in these answers) and television, we move less, and this certainly affects our health.

Interestingly enough, some dietitians have repeatedly argued that it is less about how much we eat and more about what we eat.  A little cheesecake or a lot of celery?  A pile of salad or a small french fries?  The balance, they say, is the key to both healthy wait maintenance, energy and metabolic rate, and keeping a healthy immune system.

A balanced diet is indeed very important as an issue in staying healthy - we need a good selection of fruit, vegetables, fibre, low fat protein and low sugar carbohydrates every day - the recommended intake for fruit and vegetables is five portions per person per day. Potatoes and fruit juice cannot account for all of these five as they provide different functions too. However, we also need to exercise enough to feel a bit breathless every day (aerobic exercise for those fit enough to undertake it) and we need to avoid health-damaging activities such as smoking,drinking or sunbathing. A combination or balance of all of these is the best answer.

It is certainly one of the most important issues, if not THE most important issue, because it is what we put in our bodies what ultimately dictates the regular functions of our body systems.

The problem with half of America definitely lies on the diets we follow. As obesity rises, so have the percentages of people with depression, cancer, heart conditions, high blood pressure, diabetes, and much more.  As a result of THAT we have also increased in drug dependency to both legal and illegal medications.

At this point in the American way of life and without a solid access to quality health services, THE best way to stay healthy would be definitely to lead a balanced and quality diet based on produce that would, in part, help the American farmer market their home product. So, to answer your question, it is quite reasonably to conclude that a good diet is, if not the most important, definitely close to the most important elements of a healthy life.

I would say that it is very hard to say what the most important factor is in staying healthy.

Certainly, a balanced diet is one of the most important things.  However, even when looking at what you eat, you can't just look at whether your diet is balanced.  You also have to look at the amount -- eating too much will be bad even if the food is balanced.

Outside of diet, there are other issues that have an impact on your general health.  Perhaps the most important is exercise.  A sedentary person who eats healthily will still not be all that healthy.

Then you have other stuff like smoking and drinking that can have specific bad effects, as can other things like sexual promiscuity or drug use.