Roark Junior - Nick Stahl
Roark Junior is probably the grossest villain out there. He's a sadistic killer and member of the powerful Roark clan in the film Sin City, which was adapted from a comic book. Nick Stahl played the character, who had to put on prosthetics to really fit the part.
The Invisible Man
Interestingly enough, there was no green suit CGI actor behind the Invisible Man in the movie of the same name. The antagonist was put in entirely in post-production. Elisabeth Moss plays the protagonist who's stalked by a ghost-like man after her ex-boyfriend commits suicide.