Biopower is the most famous one, but I actually think his greatest contribution was to make philosophers pay more attention to the ways in which epistemic systems and ways of organizing knowledge are connected to political power.

I actually think his phd thesis "the history of madness" is his best work. It encapsulates much of the subject matter that would occupy him (knowledge and power) in a domain that's easier to understand than some of his later arguments, and it predates his adoption of a more contorted literary style (or maybe the translation is just better, idk).

Ian Hacking also has a great text that extends Foucault's work "Historical Ontology" that picks up many of the chief ideas in a far more lucid manner for those of us who aren't fans of the later continental style (which if I'm being honest, was always a little too concerned with being obtuse just to sound intelligent)