I agree with sanders that day-to-day practical halacha should be the "default" starting point. But I would expand that thought: each person is different, each kiruv situation is different, and the best starting point is the one that will engage that particular person at that particular time. As it says in Mishlei 22:6, "Train a child according to his way ..." (h/t @IsaacMoses).
For some that way will be the basic b'rachot of daily living. For some it will be saying and starting to understand the sh'ma daily. For some it will be torah study with a good chumash. For some it will be communal ritual like the Pesach seder. For some it will center on their children.
You need to evaluate each situation -- ask probing questions, pay attention to how the person responds as you proceed, and adjust course as necessary. I've found the questions that people ask can be illuminating, revealing either an area of interest to explore or a misconception to address.
Per Avodah Zara 19a, a person can only learn Torah properly when he is learning a topic that interests him at the time (h/t @Fred). Find that topic and proceed.