I know this will be very controversial but I am ready.
So the contenders for me will be Doflamingo , Johan Libert , Meruem , Griffith , Askeladd (without order)
They are all complex villains , and for me they are all nearly equal in terms of quality but I will say why I chose Doflamingo.
Talking about Doflamingo , a villain whose story pushed the whole story of one piece to a new level , and added more depth to the politics of one piece , the most special thing about him is how detailed his story is , every Doflamingo's scene added more to his character ,
when we saw his flashback as a kid , it felt like the whole story and politics of one piece was building for his character , we saw how the nobles were raised , how they were seperated from the commoners , and how Doffy were taken from this paradise and how he developed after that . and how people around him was involved in this development.
So my main points here are the extreme details and the connection with the whole story and the world of one piece.
Johan has something like that , detailed story but I think it lacks some details especially in Johan's way to manipulate people , but he is a perfect villain.
Meruem was a different experience , it was like a story of a kid born was 200IQ exploring the world , his development was great but Doflamingo's connection to the whole world of one piece made him slightly better .
for Griffith , I haven't finished the golden age arc yet , maybe after i finish Berserk he will be better, I don't know ,but I chose Doflamingo over him relatively.
Askeladd is the closest one to Doflamingo in terms of characteristics but he didn't have the same tragic past of course . but I think Doffy overs him by details too .
these are slight differences , but I think they are worth mentioning because sadly many people don't appreciate the great writing of Doflamingo because they just ignore it.
I want detailed answers please.