Let’s say that you are talking to a Protestant about Catholicism (he isn’t militant). He knows his stuff though. Where would be the best place to begin?
Why you love it.
Then, no matter what is discussed, it seems always to end up on Authority/Apolistic Succession.
The Early Church Fathers
The Canon of Holy Scripture - who, how, and when it was put together.
If Jesus was God and you believe that, why on earth would he found the wrong Church???
I always want to start with the Eucharist. It is the source, center, and summit of our faith. If it can be biblically demonstrated, all of the other doctrines / dogmas will fall in line. Praise Jesus, it is the most biblically defensible of all our beliefs. Once it is believed in, it’s hard to find a reason to stay Protestant.
Just my thoughts.
Discuss what you have in common - a love for the Lord, reverence for the Scriptures, and what you love about being a Christian. If he or she cannot see you as a Christian for the get-go, you’re starting off on the wrong foot. If you start of with controversial topics, it sets an adversarial tone for your relationship that is difficult to overcome.
Set them at ease by finding common ground. The specific questions will come soon enough.
The sacraments and how each one was given directly to the Church from Jesus.