Karl Marx and Friedrich Engels [1818-1883 and 1820-1895]
A radical without in-depth knowledge of these two revolutionary thinkers is like a physicist who hasnt read Newton or a biologist who hasnt read Darwin. The following are some of their most essential works:
Early Writings [1840s]
There are several collections of Marx’s early writings. The most important of those writings are the Introduction to a Critique of Hegels Philosophy of Right, the Theses on Feuerbach and the Economic and Philosophical Manuscripts of 1844. In these and other texts of this period Marx and Engels are clarifying their own views by working out their differences with contemporary philosophers, economists, and radicals.
Marx and Engels,
The Communist Manifesto 
Here their fully developed radical views are first presented in a dramatic public form. It should go without saying that the communism they advocated has nothing to do with the so-called Communist regimes of the twentieth century.
[Rexroth essay on The Communist Manifesto]
The Eighteenth Brumaire of Louis Napoleon
Marx virtually invented the genre of dialectical social analysis. This examination of the 1851 coup dÉtat is the most brilliant example. See also Marx’s The Class Struggles in France: 1848-1850 and The Civil War in France (about the Paris Commune of 1871), and Engels’s Revolution and Counterrevolution in Germany (1848).
Capital (Vol. 1)
Marxs magnum opus. The first volume is the most generally important and accessible.
Socialism: Utopian and Scientific 
Popular short work derived from three chapters of Engelss book Anti-Dühring. The latter book, incidentally, is not read as much as it should be. Eugen Dühring was a pompous and fashionable academic socialist who would now be totally forgotten were it not for Engelss critique of him. This might seem to make for very boring and irrelevant reading. On the contrary, making a detailed critique of Dührings works provides a convenient framework enabling Engels to present, by contrast, his own views on virtually every important social topic. Even if Dührings particular positions are now old and forgotten, many present-day ideologues continue to spout the same general types of pretensions and illusions.
Franz Mehring, Karl Marx
This is the classic biography. There are several more modern ones, but bear in mind that modern biographers are often irresponsible and sensationalistic, and in this particular case there are many possible axes they may be grinding.
V.I. Lenin, The State and Revolution
Despite his authoritarianism, Lenin was a keen dialectical analyst who was in many cases more lucid than his opponents. In The State and Revolution he recapitulates what Marx and Engels actually said about the state, as opposed to the diverse misconceptions of their views propagated by social democrats and anarchists (and as glaringly contradicted by Lenins own bureaucratic practice once he got his own hands on the state). As such, this text merits careful reading, both for its insights and for its flaws.
Rosa Luxemburg [1871-1919]
Of the more or less orthodox Marxist thinkers, Rosa Luxemburg is perhaps the most sympathetic, not only for her life as a revolutionary but for her stress on the importance of grassroots popular activity as opposed to the hierarchical type of party organization advocated by Lenin. (The errors made by a truly revolutionary labor movement are historically infinitely more fruitful and more valuable than the infallibility of the best of all possible central committees.) There are several substantial collections, including Selected Political Writings (ed. Dick Howard), Rosa Luxemburg Speaks (ed. Mary-Alice Waters) and The Rosa Luxemburg Reader (ed. Hudis & Anderson).
History and Class
A key link between Marx and the situationists. Rather difficult in places, but one of the richest works of radical theory. Lukácss subsequent works, though sometimes interesting, are marred by his capitulation to Stalinism.
Karl Korsch, Karl Marx
This book is probably the best introduction to Marxs thought. It very lucidly cuts through several common misconceptions. The complete book, which has long been out of print, is now online at this website. Just about everything else Korsch wrote is also worth reading. Marxism and Philosophy addresses some of the same rather subtle types of issues that were tackled by Lukács. There are many other articles and essays, including a large selection with commentary: Karl Korsch: Revolutionary Theory (ed. Douglas Kellner). Here is one of his articles, with links to some of the other online material.
Raya Dunayevskaya, Marxism and Freedom
Dunayevskaya was for some time Trotskys private secretary during his exile in Mexico. She broke with him in 1939 due to his clinging to the delusion that Stalins Russia was a workers state (albeit an inexplicably deformed one) after it had become increasingly clear that it was in fact nothing but a new variant of class society: state capitalism. In Marxism and Freedom Dunayevskaya stresses the libertarian and humanistic aspects of Marxs thought that had been buried by Stalinism, Trotskyism, and economism.
Guy Debord, The Society of the Spectacle
Chapter 4 contains some penetrating analyses and critiques of Marx and the various currents of Marxism.
Section from Gateway to the Vast Realms (Ken Knabb, 2004).