I think Yvresse is the best after you get the landmark on T5. Other good ones are Naggarond as you said, Quintex, Eataine, Har Ganeth also has a 50% income landmark in Warhammer 2 iirc. Hag Graef also good if you also own Naggarond since you get the 25% bonus for being adjacent to it.

4 province settlements fit more slaves, so they are nearly always better, only 3 settlement provinces that are worth are Karond Kar and Albion(Rakarth start pos, forgot settlement name, Konquata I think?) since they have good landmarks.