Socialism: Utopian & Scientific - A great introduction into socialist theory, and the difference between the utopian socialisms and scientific socialisms. Most importantly, details and explains dialectical materialism. This text is incredibly important as it sets up the program of a large majority of the following texts.

The Origin of the Family, Private Property and the State - Really does well to show just how deeply our productive social relations define our personal social relations and the norms that we abide by.

Communist Manifesto - A timeless short pamphlet that still does well to outline Marxist thought to this day.

State and Rev - The seminal text for any Marxist Leninist. Opinions on this text will likely determine which branch of leftist thought that you fall into.

Imperialism: The Highest Stage of Capitalism - A thorough analysis of capitalism's evolution with regards to imperialism and the condensing of operations into monopolies. Capitalism has since evolved, but this will give you a very good understanding of Marxist philosophy with regards to capitalism and its many forms.

Wretched of the Earth - An analysis of revolutionary potential of settler colonial societies, really hammers home the fact that class is merely a portion of the current hegemony, and that settler colonialism and race are just as important to the contradictions as they stand

Settlers - A detailed history of settler colonialism and the unbelievable crimes Europe has committed upon the rest of the world (much like Wretched of the Earth)

Blackshirts & Reds - Looking at the 20th century from a Marxist-Leninist perspective, revisiting American anti-communism

EDIT: Currently reading The Long 20th Century as well as The Dawning of The Apocalypse (Gerald Horne).