In a post about Indonesia's energy policy, King Banaian at SCSU Scholars comments on what he thinks is the most important concept to be taught at the Principles level in economics:
When I'm asked what the most important concept in economics is, most people expect me to say supply and demand. I actually answer comparative advantage.
Agreed. In my undergraduate studies, I never heard the term comparative advantage (we never studied international trade in my courses). I never heard the term until I was well into the graduate program at the University of Nebraska-Omaha. But now, since a primary focus of my Principles of Microeconomics course is to show students how voluntary private exchange is the true workhorse of any well-functioning economy, and since exchanges between people are largely based on comparative advantage, comparative advantage is one of the first things we cover - and I cover it in class instead of production possibilities.
Indeed, if it weren't necessary for students to have been exposed to the concept of opportunity cost before they learn about comparative advantage, I'd teach opportunity cost second.
Update: Stephen Karlson over at Cold Springs Shops does similar things in his Principles class (thanks for the trackbacks, Stephen!).
I begin with voluntary exchange of already existing goods (Tom Sawyer is in the supporting cast) and introduce production by way of voluntary exchange between producers who would otherwise practice autarky. (I borrow heavily from Alchian and Allen here; Robert Frost and Socrates get involved.) Hence, the First Principle of Social Organization: There are incentives for the low-opportunity cost producers of goods to specialize in those goods. (Thus, students have a logical basis for criticizing the use of firemen as leaflet distributors as well as a way of resolving the paradox of a rich country importing food from a poor country.) Not only that, the production possibility frontier is a logical extension of opportunity cost and comparative advantage. At Northern Illinois, most of my price theory class will take income theory next.
One other thing to note: my micro students get exposed to the production possibilities frontier through a reading assignment.