What are some of the must know linguistic theories for any linguistics student?

Add to Andrew Noe’s answer:

• For historical linguistics, Uniformitarianism. (Yes, I know the link describes the geological version of that hypothesis.) The notion that human language in the past worked pretty much the same way as human language works now.
• For structuralism, as an underpinning of how we do linguistics in general: the Arbitrariness of the sign: the fact that language is mostly autonomous of the things it describes.
• For syntax, if you learn nothing else, configurationality: the notion that phrase structure rules work to describe the syntax of language, that words group together to form distinct constituents. Especially fun because of the contortions syntacticians go through to account for Non-configurational languages.
• For pragmatics, Speech act theory, accounting for language not as a mere conveying of meaning, but as agents trying to get things done in the world.