Your favorite single episode characters
Who are your favorite single episode characters from the franchise. I'll let you cheat if they've only been in two especially if those appearances were spread out over multiple series.
Gul Dar Heel from Duet is an obvious choice. Captain Phillipa Louvois from Measure of a Man. Kashyk from Counterpoint because its one of the best uses of single episode romance in the series (kind of a low bar to clear I'll grant you.)
I'd be remiss if I didn't mention Hugh. Granted he was in a couple of episodes (three if you count Descent as two episodes) but I'm sure he was originally only intended for one. He's a good study for people who are prone to writing Mary Sues. He's there to help us explore our main characters, not to steal the show.
Moriarty is another one who is more interesting conceptually than he is as an individual. But the actor played him well. And he made a good adversary for Data, as he was designed to be. He did so even in spite of transcending that design, making him an interesting parallel to Data.
What are your picks?