I like to recommend Wage, Labor, and Capital by Marx (my physical copy also contains Value, Price, and Profit). It's short, written to be understood by 19th century working class Germans, and is sort of like Capital volume 0.1.
Otherwise, State and Revolution or What is to be Done? by Lenin are bangers, and I like to recommend them to ML-curious people (usually people who have just taken the first steps away from just being a radlib faux-anarchist). Lenin was a great writer, engaging and passionate, but his works build on a large background of Marx and Engels (and others, of course), and you might not get all of the references.
EDIT: In case you didn't know yet, basically everything written by Marx, Engels, Lenin, and a lot of others are available for free on Marxists.org