Definitely Marx and Lenin. They will give you a good overview of what marxism-leninism is about and why it’s doomed to failure. I don’t advise reading Trotsky on the other hand because that one claims to be a victim of tyranny fighting for freedom, when he is (was) the tyrant himself so the message a newcomer without historical facts would receive is “Trotsky is a hero”, whereas a person reading “Lenin” will sonner or later read through his socialist claims and find out he’s a complete power-hungry psychopath.