For me, there are two that stand out in my mind as being equally effective and bearing the most influence on my life. The first is courses in interpersonal skills. Learning how to speak and respond to others in a way that preserves human dignity. The second is a seminary course I took in Christian apologetic. Of all the apologists, former atheist CS Lewis' moral proof for the existence of God is the most compelling. His book, "Mere Christianity" is awesome.