I do believe DS9 had the best ensemble of actors. If you look at the principal cast, I can't think of a single bad actor. Part of that I think has to do with the fact the DS9 cast had the best material with which to work DS9 compared to TNG or VOY was a far more character based show. I think the TNG cast was great too but a lot of the actors didn't have the opportunities to shine often.
Yeah, DS9 devoted a lot more time to character development than the other shows. It seemed like even the supporting characters, like Garak and Nog, got more development than Geordi or Harry Kim.
That being said, I do think DS9 had the highest number of talented actors. Armin Shimerman, Rene Auberjonois, Nana Visitor, and Colm Meaney were always fantastic. Shimerman, in particular, never failed to amaze me with his acting. Whether he was being funny or serious, typical greedy Ferengi or unlikely hero, he was always Quark. And it wasn't just the makeup.
Actually, don't kill me for saying this, but Avery Brooks always came off as the weakest actor out of the main cast.