•Why it earns a slot
The foundational text of Western political science, built by studying 158 real Greek constitutions rather than reasoning from pure theory, and the source of the claim that humans are inherently social and political creatures.
Aristotle's Politics starts from a single claim: the city-state (polis) is not an artificial contract people opted into, it is the natural endpoint of human social life, and a human being cut off from it is 'either a beast or a god.' From that foundation he builds an analysis of family, household, and the different ways governments can be organized well or badly.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.