State and revolution.

The State and Revolution was incredibly important for my political development. It was the work that plunged me into a Marxist worldview. One of the main realizations that I remember from reading it was the idea that the state isn't just a neutral entity that represents the interests of all classes in society, but rather, it's an instrument of class rule that suppresses the will of the working class to the benefit of the capitalist class (under capitalism). That idea clicked with me and transformed my worldview.

The State and Revolution is a pretty good introduction to Marxism.