6. Captain Maria LaGuerta - Season 7
Though Maria LaGuerta was a recurring cast member since the pilot episode, she didn't become a foe of Dexter's until the show's seventh season. While investigating the crime scene left at Travis Marshall's church, she comes across a blood slide left at the scene. This prompts her to start re-investigating the Bay Harbor Butcher case, which everyone thinks is closed thanks to Dexter's framing of Sgt. Doakes. We never really got a glimpse of LaGuerta's detective side until season 7, and I believe it was handled quite well. In the past her character always seemed a bit annoying, and any story revolving around her wasn't very interesting or important to the larger themes at hand. The writers truly gave her some great moments in season 7, including how well she manages to figure out Dexter's secret, and Deb's involvement in it, before ultimately paying the price. Though writers have (at this point, at least) failed to truly capitalize on her death in the season 7 finale, her last few episodes delivered some of the most exciting, intense moments of the entire series. She definitely went out with a bang.
Memorable Moment: Arresting Dexter for being the Bay Harbor Butcher.