For me it hast to be Duet
, before this all the previous episodes strayed into well-trodden TNG territory or were too timid and didn't make bold leaps forwards.
changed all that, for starters it focuses solely on two alien characters who drove this episode forwards, and TNG has NEVER done that before. Secondly there is a level of psychological and emotional insight that TNG only occasionally achieved. TNG sided with the humans one way or another, but in Duet
you begin to understand just a fraction of what the Bajorans went through, you sympathise with someone other than the humans which makes a nice change. Kira's passionate account about the liberation of the Gallitep labour camp is so powerful because she descends into near hysterics.
There are some seriously raw emotions, and the 'villain' of this episode, the disguised Marritza, you almost want to hear more of his brutal and frank opinions because the episode isn't biased towards him or Kira. It's a clash of opinions, of facts and of accounts, and it's allowed to happen and it's allowed to run its course. Finally cumulating in what I see is a major breakthrough for Kira's character; she finally differentiates between those Cardassians who committed all those terrible things against her people, and the ones who did not.
It's classic Roddenbury stuff, except it's between two alien species and the episode is totally unrepentant and in the greater scheme of things the schism between the Bajorans and the Cardassians still remains, but this episode tells us that maybe there will be reconciliation but it will take a hell of a long time before this happens (i.e. the Bajoran who murdered Marritza).
For me Duet
was when DS9 truly showed its potential; it started to really focus on the people, and move away from this planet-of-the-week, travelling-through-the-stars format which TOS and TNG were so heavily bound to.