Jason Fried says his favourite economic precept is 'sunk cost'.
I think sunk cost is mostly interesting because of the prevalence of the sunk cost fallacy where people say things like "I don't feel like going to the concert but I've already spent the money on the ticket so it would be a waste not to go".
This fallacy, like many in economics, comes down to not appreciating opportunity cost, particularly applied to that most precious of resources: time.
Another favourite economic precept of mine is comparative advantage. Comparative advantage is key to understanding why voluntary trade is win-win, even when one party can produce all the goods most efficiently. At its core, though, comparative advantage is a corollary to opportunity cost.
Yet another favourite economic precept of mine is Frédéric Bastiat's broken window fallacy where people say things like "at least this natural disaster will be good for the construction industry". But again, the broken window fallacy is about (not) understanding opportunity cost.
So I would have to say opportunity cost is my favourite economic precept because it is foundational to so many other important precepts.
Of course, I've mentioned before on this blog that I also think marginal utility and the subjective theory of value are very important to understand.
If I made a list of "100 things everyone should know" I think I would put all the precepts mentioned here on it.