I feel Kilgrave is in a league of his own in terms of being the best villain in the entire Netflix continuity bar nobody else. Cottonmouth and Kingpin are both in a very strong second place, but they just didn't have the same disturbing gravitas Tennant brought to his performance as the Purple Man.
Kilgrave just got to me as a villain, and it was very much because Tennant made it work masterfully. A guy who could make you do anything he wanted you to do by just telling you to, even forcing an entire precinct to put guns to their own heads just to mess with his own object of interest, Jessica Jones. And of course the very much implied undertones of rape go a long way in making you just straight up despise Kilgrave, who abuses his powers because ultimately he's just a bored petulant manchild who only gets his kicks from using people.
Kingpin is also brilliant, but due to the character's general 'silent fury' aspects he feels a little bit underdeveloped at times. But D'Onofrio seems to be made for the role, and he's just perfect as a foil to Daredevil. A ruthless criminal who has machinations of being more, a guy that wishes to be remembered, to be a politician and basically run New York both under and above the table. He's clearly a dangerous man, but it's all a lot more focussed, controlled, but of course when he kills it's aptly gruesome and shocking.
Cottonmouth is very much like Kingpin, in that he's a gang leader that wishes to be more. Who wants to be the king of Harlem, more like an Avon Barksdale to bring The Wire references into it. A man who wants to be a gang leader, a beloved 'classic' gangster who gets respect through fear/control. But his sister is the wannabe mover and shaker.
I liked Diamondback and Harold Meachum are a bit more on the sillier side. Diamondback ventures straight up into saturday morning cartoon villainy the way he chews the scenery, and Harold is unhinged and crazy because of his resurrections.
Also very big mention, Madame Gao. A character you know is dangerous even though she looks like a pleasant old asian lady, unassuming danger.