5. The Judoon
When introducing a new companion, a good episode storyline lacks as much as a decent alien does. "Smith and Jones" was an okay episode. It delivered what was expected, giving us an insight on the new companion's life. This, of course, meant less screen time for an alien race. When they appeared in "The Stolen Earth", they were utterly useless to the story. And when in the Sarah Jane Adventures, they were portrayed as trigger-happy bimbos without any authority or morals. They're one-off creatures that should stay that way. They lack everything a decent villain had. To have them in another episode would be a bad idea because the story would lack a good adventure. Which leads me to...