define successful in your mind? The one i am most familiar with, and lasted several hundred years was the Parthian empire. Which had many independent kingdoms whos kings elected a "king of kings" who technically ruled the country as a whole, and all other kings paid tribute. However his power was only as good as his supporters, and often the kings of the more powerful kingdoms would have heavy influence or even outright defiance. The King of kings would handle tax tribute, had only a small kingdom (city) to himself and really could not declare war or implement changes without the consent of the major kingdoms.

This system lasted for a couple hundred years in Parthia during the Roman expansions. But it was rife with civil war and conflict. I guess you could call it successful during peace years when they had a strong king, and they often united in face of external threats. But I'm not sure I would qualify it as wholly successful.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.