Star Trek: Deep Space Nine is unique in the franchise in that the main location was stationary for the seven-year run, as opposed to flying around the Quadrant on a starship. That isn't to say that we didn't travel to other worlds, meet new aliens and avoid Dominion territory like the plague, but in this series, the guest stars tended to come to the Station, rather than the other way around.
That meant there were dozens of episodes that focused on characters who weren't part of the main cast. In fact, Deep Space Nine probably spent the most time out of the entire franchise telling the stories of those people who weren't listed in the opening titles. The likes of Garak, Dukat, Damar, and Kai Winn, love them or hate them, would never have had such a chance to breathe on The Original Series, The Next Generation, Voyager, or Enterprise.
There are the enemies who became better developed than some of the Starfleet giants, along with allies who truly stepped up to the plate. This series knew how to tell a serialized story with many different players and these are ten of the best examples.
10. Soldiers Of The Empire
General Martok had been introduced to Deep Space Nine in stages. First, there is the Changeling version of him that encourages Chancellor Gowron to go to war with both the Cardassians and the Federation. When this copy is later killed, he is reintroduced as a prisoner at Internment Centre 375. This version of the man is grizzled and bashed but very much the honourable Klingon he is expected to be.
Soldiers of the Empire sees him given command of a vessel, the IKS Rotarran. This ship is ordered to search for the B'moth, a Klingon battlecruiser that may have drifted into enemy territory. Then, Martok's struggle is revealed: he is dealing with the PTSD of having been imprisoned and beaten by the Jem'hadar, daily, for months. They have, in effect, broken him.
Worf sees this, struggling with loyalty to his friend and the growing discontent of the Klingon crew. As Dax puts it, the ship needs a big victory, or a mutiny may be on their hands. Worf challenges Martok, correctly assuming that direct confrontation is what is needed to shake the General from his doldrums. Reenergized, the B'moth is saved, the crew rejoice and the episode ends with a wonderful Klingon sing-along.