You've got a bit of a choose your own adventure!

If you consider yourself a liberal and generally against AES like the USSR, PRC, Cuba, etc, Blackshirts and Reds is a fantastic critical reexamination and reads very well. Nothing but constant truth bombs.

If you want to get into Marxism, I recommend The Principles of Communism followed by Socialism: Utopian and Scientific as well as Elementary Principles of Philosophy. An intro/FAQ of Communism, followed by the history of Socialism and how and why Marxism answers the problems with previous Utopian Socialists, and finally the best work on the philosophical aspect of Marxism, Dialectical and Historical Materialism.

If you want some quick reads, I love Why Do Marxists Fail to Bring the "Worker's Paradise?" as well as Socialism Developed China, Not Capitalism. Modern analysis, 20 minute reads, based on what we currently know and not written back in the period of Marx.

Finally, if you consider yourself a Marxist already, The State and Revolution as well as Imperialism, The Highest Stage of Capitalism are both Lenin's most significant works.

Really though, the modern works and Blackshirts and Reds are great primers before delving into Marx, Engels, and Lenin themselves.