‘Efficient’ in what sense? What is meant by "works best"?
Dictatorships are certainly ‘efficient’ at producing and maintaining failed states. Most dictatorships in existence today are relative economic basket cases. And name a former dictatorship in recent history that didn’t cause chaos and pain for its citizens.
According to R.J. Rummel in "Death By Government" dictatorships were responsible for hundreds of millions deaths during the 20th century. So I suppose that makes them ‘efficient’ killers.