I've been searching about this for a while, I have been looking for historical rulers who based their way of governing off Buddhist principles. But haven't been able to find a lot.
A good example is Asoka who ruled most of the India subcontinent between 269 BCE to 232 BCE. He converted to Buddhism after witnessing the slaughter at Kalinga, a slaughter that he was responsible for. A rock inscription made at the time details this.
His Majesty feels remorse on account of the conquest of Kalinga because, during the subjugation of a previously unconquered country, slaughter, death, and taking away captive of the people necessarily occur, whereat His Majesty feels profound sorrow and regret.
Asoka went on to rule for forty years on Buddhist principle and was one of the main people responsible for the initial spread of Buddhism out of the Indian mainland. He sent monks to many places throughout Asia and beyond including Mongolia, Burma, Afghanistan, Persia even as far as Turkey and Italy.
Such is his enormous repute they even made a Bollywood film about him. I have to confess that I've never seen it. I would be interested to know if it is worth the watch.
BBC radio have just done an excellent program on Asoka as part of the (also excellent) In Our Time series. Very much worth a listen if anyone is interested in this part of Buddhist history.
Asoka. But we know so little about what he did. His pillars say he introduced Buddhist-influenced, incredibly progressive public policies.
Emperor Wu of Liang- But I'm having a hard time finding good sources, I guess they didn't keep great records in 500CE It sounds like many of the stories about him were embellishments. ref: http://khyentsefoundation.org/2011/02/patron-kings-part-xii-emperor-wu-of-liang/
Empress Wu- She wasn't a very nice person though.
The Dalai Lamas of Tibet.
All of these leaders were old time leaders who presided over a state that did mean things. I think people care about this question because they want to know if a Buddhist government has ever been for the better. To know for sure, we'd need the counterfactual, of what would Tibet have been like if Buddhism didn't appear? If it weren't for Buddhism, would Asoka have done anything different?