Tough call. The Klingon ones were certainly the most involved & developed. The Borg ones weren't so much, but they did have a whole lot of potency, but I guess the ones I value the most are the Romulan & Cardassian ones
By and large, the Cardassian episodes, what few of them there were, managed to instill a particularly gripping realism that had alluded TNG prior, but there's so few that really center around them. There's more that just use them, like Preemptive Strike or even Journey's End & Lower Decks. Chain of Command & The Wounded are really the only ones that focus on them
The Romulans, on the other hand, have a good deal more, that are TNG's best, The Defector, The Enemy, Face of The Enemy, Unification, The Next Phase, & even Timescape. I'd say the Romulans got a more varied treatment. So I'd probably have to say they're my favorite