^^ You've suggested bias in others yet yours is clearly showing. The acting in DS9 is in no way whatsoever superior to what has been seen in TNG and TOS and many other television series.
In TOS the secondary characters suffered from lack of exposure and sufficient materiel to work with. And that's also been true in many other series. I've seen plenty enough of DS9 to see that the acting there can leave a great deal to be desired and that is from the main characters.
James Doohan and Nichelle Nichols in particular have done acceptable work elsewhere besides TOS, but they're not well known for it. Same with Majel Barrett. But in TOS they were rarely called upon to do much. particularly after the first season. I will say that I like Scotty much better in the first season because he's much less caricatured, but from Season 2 onward and into the films (except perhaps TMP) he became more exaggerated and caricatured. Same with Walter Koenig as Chekov. In this I blame the direction they were given rather than what I think they were capable of. With Nichelle Nichols I do know that if you watch Uhura in the background she's always doing something rather than just waiting to say a few lines. I also think the TOS cast, both regular and guest, were often quite good with non verbal acting---they could telegraph a great deal with their eyes, body language and facial expressions. Nimoy had to be particularly good at this to convey so much from a character who claimed to have no emotions.
You can say you really like the cast of DS9, but objectively they were not in anyway superior. Indeed all the Trek series have had decent casts, even the shows I detest (VOY and ENT), and so it really came down to what was asked of them. It came down to the writing and direction.
Fair enough. Then I guess DS9 just had better writing and direction.
Actually, in all seriousness, I do disagree with you, slightly. I think all the Trek shows had at least a couple of classically-trained superior theatre actors that stood head and shoulders above the other actors in the cast. In TOS, Shatner, Kelley, and Nimoy in particular, are clearly the best actors of the bunch, while on TNG, Patrick Stewart is obviously wiping the floor with everyone else's asses (except perhaps Brent Spiner who, although pretty bad at other parts, is excellent as Data.) It's not just writing and direction - Nimoy, when given great material, does more with it than, say, Doohan, when given good material. And Stewart is better, when given any material, than, say, Jonathan Frakes, when given fantastic material. Some of the actors are clearly better than others.
My opinion about DS9 comes from the fact (I believe) that there are more of those stand-out actors on the show than on the other shows. Colm Meaney, Nana Visitor, Rene Auberjonois, Mark Alaimo, Andrew Robinson...there are just more classically-trained theatre actors on that show, and they do stand out above the more mediocre television actors. I just think a higher percentage of the DS9 cast can hit great material out of the park than on the other Star Trek shows.