According to research, the most important personality trait when it comes to health, healthy living, and longevity is?

Conscientiousness.

Explanation:

Conscientiousness is personality trait that makes individuals to act carefully and diligently. Such people approach any task with great attention to details, desire to follow the process through and not to slack off on any step.

They take all matters seriously and responsibly, which equally concerns health. For example, a conscientious person would take all necessary vaccines before traveling to areas with high risk of contracting infectious diseases, such as South-east Asia.