Who was the best one-off villain from the original series?

[Herbert]

You mean after Kirk rams the Constellation down its throat.

No, that was after the chili

 

[Spock's Barber]

Nomad "The Changeling"

Security Chief : "Captain Kirk, Nomad is being naughty again. We're going to a few need more red shirts on board the Enterprise."

 

[F. King Daniel]

I know it's become fashionable to consider Trelane to be a Q but I'm calling BS on that. What Q ever need an apparatus to augment his power?

"The Q and the Gray". Muskets in the continuum which caused supernovae in the real world.