I thought DS9 did a spectacular job with the Ferengi and Cardassians, but a very poor job with the Bajorans. Still don't a good grasp on what they're all about.
But in general I agree--the Cardassians. We learned a great deal about them, not only in terms of detail but actual depth. It all fit together into a whole. Like what Cardassian mysteries are like. And how they look upon family. I believed in them.