Star Trek: Deep Space Nine

DS9 is more character-focused and has longer story arcs than any Star Trek series before it (and after it until Disco). Unlike Disco, which is sheer melodrama, DS9 manages to simultaneously be fun and challenge viewers with dark and thought-provoking content. It's a difficult balance to achieve, and the writing team deserves kudos.

The interactions between Odo and Quark, Bashir and Garak, and Sisko and Dax are the biggest highlights. Dukat is one of the greatest Star Trek antagonists ever, and is played masterfully by Marc Alaimo. All of the characters feel like real people, including the Bajorans, who are immensely frustrating with their religious doctrine and political pursuits.

Unlike other Trek series, it's hard to think of standout episodes of DS9 (except maybe "The Visitor"), but the series as a whole is very well done.