This.

Lenin is the means of understanding how to put any of that into practice, as well as a generally more readable writer. Plus more emphasis on the modern forms of liberalism and what social democracy would become, and an understanding of internationalism and self-determination that Marx and Engels never really managed. Keep in mind how Ho Chi Minh found the answers to every observation and deeply held feeling he had just by reading some Lenin. Lenin makes sense of it all in a way no one else does