One thing for me was how light hearted and funny Lenin was. I expected it to be really hard going like a lot of Marx but he had such a wit about him and I remember laughing out loud a few times by his turn of phrase. I don’t think he gets enough credit for it, he took a really serious, dry subject and made it enjoyable to read. He really was an incredible writer.
I’d say most of The Critique of the Gotha Program. Like baby leftist me would have been like “hell yea every group that ain’t the Revolutionary proletariat is one reactionary mass of enemies” or “yea the working class needs to liberate labor and give undiminished proceeds of labor straight to the worker! Everything for the worker first!”. Reading that critique really improved my understanding of how socialism works and how not to implement it(with idealist goals)