What are the most beneficial and cost-effective preventive services?

According to a study performed by the National Commission on Prevention Priorities they are:

1. Aspirin chemoprophylaxis
2. Childhood immunization series
3. Tobacco-use screening and brief intervention

See the full results of this study at the AAFP website "Practice Guidelines:
Partnership for Prevention Ranks Preventive Services." Be certain to check out Table 2 which depicts current utilization rates (the current tobacco-use screening and brief intervention national utilization rate is only 35%) and the additional quality-adjusted life years saved if utilization rates were at 90%.