In spirit of the post, Wage Labor and Capital, Value, Price and Profit, and Critique of the Gotha Programme are the easiest to digest works by Marx, and the most applicable to general leftism. All are short reads too, each can be completed in around an hour.

Engels has some great works too, and his works are generally grouped in with Marx’s because they worked together closely, but I kept it to strictly Marx.

I think the Silo books were pretty good