It depends what the reader is familiar with.
1. For those that have been in a more sheltered yeshiva life, try Nesivos Shalom. It has the yeshivish flow, and knowing chassidishe terms is not a prerequisite.
2. If the reader is in yeshiva but has been more exposed to general thought, Rav Tzadok is a must. As he himself started rom the peak of the Torah world.
3. Rav Nachman is very popular now with yeshiva boys struggling to keep up. It used to be Noam Elimelech.
4. Yeshiva guys that know Nefesh Hachaim very well, would find Tanya very enjoyable. But learn it yourself. And definitely not with a Chabadzker. But if the goal is to leave the yeshiva world behind, than by all means find yourself the most committed Chabadzker out there! Just be prepared for plenty of fireworks…
5. Someone who already has developed a strong meditational side, could go straight to the Bais Yaakov of Radzin.
6. Sefardim should use Bnei Yissasschar. An old Hungarian yid, told me that the author intentionally wrote with a less European and more Mediterranean style.
7. Chassiddim that want to take a new look at Chassiddus, must see Vealeh Divrei Dovid Haacharonim.
8. If the idea is to study real Chassiddus in a detached fashion, read Aryeh Kaplan’s books.
9. Also, the Chofetz Chaim wrote many works that are parallel to chassidus.
10. And if you want the knowledgeable experience without the effort or commitment, there is always Shlomo’s Stories?