Two texts by Seneca: "On the shortness of life" and "On Providence". The first one made me rethink the idea of "productivity" and the second one made me better at handling bad situations. But at the time I still felt crushed below the weight of a meaningless world, and then I read "The myth of Sisyphus" by Camus and my mind was blown. It was such an inovative way to deal with a world that doesn't answer back.

Also "Discourses" of Epictetus. If there ever was a book that was simple, elegant, and usable right away for a better life, this is it. I'd recommend this to everyone.

It's hard to single out specific works of Plato to stand on their own, I find the most value to be gained by having an overview of his whole philosophy, but "Protagoras" is my favourite dialogue, as it introduces some essential questions as to why are we so careless when taking care of our minds, and how nobody does bad things willingly (which is often repeated by Epictetus). Also the "Apology" is essential because it shows the basic thoughts that guided the greatest philosopher in the west.