"Is Bondrewd the greatest villain in the current anime/manga genre?"
No, he's not by the very fact that he's self-aware and accepted by his society. He isn't evil because he does the things that he does for the sake of research and has been beneficial to the understanding of the world in made in abyss. He's the nightmare of progressiveness, he doesn't consider ethics or limitations, bondrewd does everything possible and sacrifices whatever it may be to gain that knowledge. His morality isn't questionable it's pure utilitarianism.
Griffith is a genuine villain because everything he does is selfish, he even hurts himself to accomplish his goals. He's consumed by his dream, there's no rationality behind him that justifies his actions other than his accomplishments. The people of the world that knows what came before him knows he's evil, and even now it's apparent that people will continue to attempt to take him down, his may be one of the final kingdoms but it was built on the sacrifice and blood of innocents across the entire world. He isn't making a better world he's making his own kingdom of heaven that is his cage for others to call him their savior and divine ruler.
They are completely different beings, one is accepted by their world and is aptly named for his enlightenment and progress of information, the other is a vile creature without equal a man made devil and just like the devil there are those who accept the embrace of demons and those who rage in spite of their insignificance in the face of his absolute terror and destruction. Plus just think about the fact that nearly everything supernatural and evil in the world is beholden to Griffith, yet Bondrewd continues needing to live in response to the mechanics and innate nature of the world.
Let me reiterate, did you read Berserk?
