Rover's final episode, for Rover himself. Not only does a slapped-together heap of Sentinel parts choose to come back and save the mutants he has been working with, pulling off an awesome Heroic Sacrifice and overriding the Kid With The Remote Control, there's also his final scene, where he manages to overpower his busted voicebox to finally say one word that isn't "Destroy": "Run!"
Arguably, Marrow's actions during that episode as well. Sentinels have been her captors as long as she's been alive, yet her connection to Rover is so obviously strong, as is her grief when he gets destroyed. Just what sort of life has she lived that she can consider a rebuilt Sentinel her closest friend?
The show's version of Nitro. Unlike his comic book counterpart who was a killer for hire, this version has no desire to hurt people and is only dangerous because he can't control his powers. At the end of his one episode appearance, he willingly returns to confinement where he is placed in a special prison to keep him from exploding.
Broo's heartbreak when Idie dances with Quentin and then getting shot.