To my tastes, I like having the stat blocks of high level rulers, with nice equipment and stuff, as well as long background text about their lives and exploits. I use them a lot in my gaming sessions. I don't need pages and pages of stat blocks for cannon fodder, I have done those to death in the past and there are loads of character generators on the net that can do the job of creating random NPCs just fine.
Now, what about NPC rulers confronting recurrent villains the PCs can't currently beat? I mean that kind of villain you keep teasing the PCs with until they grow to a specific level.
Rulers are supposed to be rulers either by their GREAT warrior skills and experience, their GREAT magic skills and experiencie, their GREAT diplomatic skills and experience, their GREAT businessman skills and experience, or some such thing. Villains come in many flavors, and the NPC rulers became rulers for beating many of them, whether those villains were businessmen, polititians, tyrans, warlords or such, and should be prepared to endure attacks of many more to come.
One of the things I do in my game sessions as a GM is have the villains confront some important NPC every once in a while and show off the NPC's skills a bit, repelling the villain's attack and thus making that NPC admirable in the PCs' eyes, giving them an idea of why they are where they are. Several sessions later, it will be the PCs' turn against the villain, once they grow enough.
I usually make stat blocks of homebrew NPC rulers with 12-14 levels, and important authorities like a sheriff, high priest or such with 10 levels.