Which of the following rush factors has the most immediate impact on a persons health ?

Eating junk foods & not exercising can cause the development of obesity which, in the "long run," can be a "factor" in the development of "lifestyle diseases" such as hypertension, hyperlipidemia and the like. Likewise, the "effects" of smoking cigarettes, like COPD (chronic obstructive pulmonary disease), will most likely manifest "later on."

When you mentioned "immediate," it's safe to assume that the "risk factor" your looking for can impact an individual with a "snap" of a finger; and accidents, as we all know can happen real time due to weather conditions, poor judgment, road mishaps and not exercising enough caution such as not wearing a seatbelt while manouvering the streets behind the wheel.