I tried Nal Tar. Once you have it, it is very isolated and good to hold. When you than take Sher Quila and Highpass Fortress its get even better. But there is the on big downside, the Dshar Raiders. They are often to quick for your own patrols and are hunting your Farmers down very quick.
Laria i made once very good experience. The downside is, that after conquering the 2 northern fortresses, further expansion is difficult to hold.
I think the Ravenstern cities as a whole a good start because its isolated too (even Ravenstern in my opinion).