If we consider "monarchy" to be a modern system...because let's face it, it is probably the oldest...where other than monarchy and republican democracy has a child following their parent ever made for a decent ruler?
Not that the odds are all that great in either Monarchy (I mean King Louis was awesome, but his son King Louis was terrible, and his son King Louis was then not so bad but then his son King Louis was the worst) or a Republic. There were Pitt the Elder and Younger in Britain, but in America we've gotten John & John Quincy Adams -- average at best and George & George W. Bush, about which you can only say, well, they weren't the Duvaliers (if for no other reason than no one would ever call the latter Bush "Doc" for any reason).
But nobody does worse than the dictatorship. From the North Korean Kims to the Hussein brothers to the Assads.