Emperor Ganishka. People are gonna start thinking I'm a fan of his (lol), but Miura really did meet whatever expectations we could have for an apostle beyond what regular apostles would be: an electric cloud. And yet, he tried to surpass that and really managed to do it. Miura went to the form beyond all expected forms, and then actually gave us something further than that succesfully. I don't see many authors daring so much without failing hard.
Picking an awful example, Kubo Tite tried to do that with the Espadas and so far nobody agrees with the power ranking he alledged they would have. Other authors simply run away from creating a character beyond the rest in every way. And yet, Miura proven to us that he's not just "God-hands>>>>>mortals", and he can think over a nice example of between that works.
A cloud is really impossible to fight against, a "Godly" entity that is unkillable by any normal means. And whatever that Hell-incarnation was, it surpassed the "cloud". A true God on Earth, which only Griffith's deus-ex-machina (literally!) could have dealt with. Add to that Ganishka's past self and his whole kingdom's theme, and we'll get quite an unique villain, that by the way we have no idea what happened to so far. lol