I'll refrain from giving 12 Byzantine Rulers the lowest possible rating because Brownworth clearly knows the subject matter and appears to genuinely love Byzantine history.
But it's impossible to ignore the elephant in the room: this podcast is deeply, openly, almost comically bigoted. It is commendable that Brownworth loves the Byzantines; what is not commendable is that he appears to hate Muslims, or at least the Islamic civilization that was Byzantium's chief antagonist during its final eight centuries.
A single review can't possibly list every example of the contempt with which he holds these Muslims over the span of 17 episodes, but here's a direct quote of something he says in Episode 17 - Conclusion:
"After all, the stoutness of its walls checked the seemingly irresistible Muslim tide, forcing the armies of Islam to take the long way through north Africa and Spain, thereby overextending their resources and giving Europe the time it needed to resist. Thomas Cahill was wrong: the Irish didn't save civilization, the Byzantines did."
If your view of the world is that the Byzantines "saved civilization" from the "irresistible Muslim tide", you must believe that the various Muslim empires were not just uncivilized, but an actual danger to civilization - this, even though the quality of life, including the level of religious, academic, and economic freedom, was far higher in the Islamic world than it was in the Byzantine empire for virtually the entire 800 years the two civilizations co-existed. And if that's your view, I would submit that you have a problem with bigotry. And that your podcast is fatally flawed.