3. Edwin Stillwell
There's smug and then there's Edwin Stillwell (Ian Bohen). The internal affairs officer was, like Woods, always trying to get Voight into trouble. His passive aggressive tone rubs Voight the wrong way when he's introduced in season 1, and he continues to be a thorn in the character's side.
Stillwell would play a pretty substantial role in the show's first two seasons, but his run came to an abrupt end in the episode "Call It Macaroni", when he arrested and charged with the murder of a cop named Sheldon Jin (Archie Kao).
There isn't a big showdown between Stillwell and Voight, but we do get a quick shot of the latter looking satisfied as Stillwell gets cuffed.