Man, so many choices; so many choices. I went with "military," simply because the cost is SO huge that it's the number one thing in the federal budget. And, once having a giant military, there is heavy pressure to make use of it. There is nothing more useless than a peace-time military, so some war or other must always be sought out.
But, the disastrous "war" on drug users is another good candidate. It's cause enormous pain and suffering, brought about a gigantic surge in crime, and ruined the lives of countless Americans. It has brought about a police state in this country.
Government schools would also be a good choice as the worst program. (I hesitate to call it "education," since doing so would imply that some educatin' is actually taking place.) These state factories ruin children's minds, with the goal of keeping them stupid and obedient to government. After 12 years of indoctrination, the young person walks out into the world convinced that all things good and wonderful flow from the mouth of the all-powerful state.