Who was Jack's greatest Villian

I've watched seasons 1 to 6, and so far, I'd also have to say Nina.

The Drazens

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Cheng and as a close second Nina.

Nina Myers, because he trusted her with not only his life on more than one occasion, but the lives of his family, and she killed his wife.

Nina is the most beloved and famous and had more character, being more important to Jack than any other enemy, although her motivations were cliched. Also she killed Teri. The biggest blow in Jack's life.

Cheng was a real pain in Jack's ass for kidnapping and torturing him and Audrey, he was indirectly responsible for her death. Also, if we think about it, he was indirectly responsible for Michelle's death too as he forced Jack into hiding which prompted the villains from S5 to lure him out by trying to kill his friends.

Logan was also great, Greg Itzin was marvelous, but they wasted him in S6 and S8. But he was golden in S5.

It's a tough one, but I will pick Nina. She was an A-Bitch.

I once told a man to go screw himself! Can you even imagine?! - Kilgrave

Kim.

Had she just gone to sleep after their chess game then his wife would still be alive.

Seasons 1-8, I'll go with Nina. She set Jack's life on a very dark path.

Including season 9, Cheng. He got Audrey killed, pretty much cementing his realization of the permanent emptiness in his life.