Felix and Locust we're an interesting pair. Normally, I'd say the Meta, but in truth he was just a bi-product of the Director who was also a good villain.
However Felix in particular was a great counter to Carolina. His personality and skill level we're great. But what made him the best villain was how crazy he really was. If you analyze the things he dies and how he behaves, it's a bit creepy.
Locus also offers as a good opposite to Wash, but he doesn't see himself as evil. He thinks he's justified, so he doesn't see the wrong he is doing.
They both offer a cool character concept to the show.