Do you count Kings as dictators? I don't really (the middle eastern monarchies have weathered the chaos in the region, if Kings were no different from dictators you would expect at least some of them to fall too...) but there is of course such a thing as a 'bad king'. I recalled this simplified analysis of Polish monarchs: https://archive.is/QEY1n tldr 250 out of 835 years of bad monarchs, or a 30% failure ratio by time. 18 individual monarchs out of 48, or a 37.5% failure rate by person. Very few 20th century dictatorships have even survived into the 21st century.
I've seen similar ratings for Roman emperors as well but confess I don't remember the details, I wouldn't be surprised if someone has further compiled information to address exactly this question for other European lines of monarchs, the various Chinese dynasties, Japan's weird history, etc. (https://archive.is/I42xd is a good reminder of the excesses, no form of government is innocent.)
I've seen similar ratings for Roman emperors as well but confess I don't remember the details, I wouldn't be surprised if someone has further compiled information to address exactly this question for other European lines of monarchs, the various Chinese dynasties, Japan's weird history, etc. (https://archive.is/I42xd is a good reminder of the excesses, no form of government is innocent.)