Negan (and Lucille)
Rivaled only by The Governor, Negan is the most brutal enemy Rick and his group have faced in the comics.
Negan firmly establishes his unequivocal villainy in the comics with displays of force that are grotesque, even by The Walking Dead
's gory standards. He perpetrates some particularly stomach-turning violence with the help of his barbed wire-wrapped baseball bat, Lucille.
If and when Negan appears on the TV series, it will be a challenge for showrunners to match the intensity of the vulgarity he exhibits on the books' pages without being able to make use of the highly colorful and somewhat limited vocabulary that does so much to define his character.
It will take some time for the shattered group on the show to get themselves on solid enough footing to stand up to someone like Negan, so it will probably be a while before he is introduced on screen, but once he is, his antagonism will likely redefine the course of the series, if the comic is any indication.
Editor's Note: This one is happening! Since we first wrote this story, the show announced Negan would arrive on the show in 2016, played by actor Jeffrey Dean Morgan.