Star Trek: The Original Series has featured some memorable guest stars alongside its iconic regular cast.

It was rare that an episode of Star Trek didn't feature any guest stars: only two episodes -- "The Immunity Syndrome" and "The Tholian Web" -- come to mind offhand as having no featured guest players in addition to the show's main stars and assorted supporting cast. And while some of those guest stars ranged from colorful if not particularly memorable aliens to generic Federation bureaucrats -- anyone remember Commodore Stocker? -- Star Trek in its three seasons also unveiled a surprisingly strong gallery of exceptional guest stars who graced the show with striking performances, ingenious characters or both.

Here then is a list of the 17 best guest stars -- hero or villain -- to ever hitch a ride on, attempt to hijack, threaten to destroy or capture as a plaything the U.S.S. Enterprise and provide its crew with that week's measure of excitement, terror or romance. They're all in their own way as beloved to Trekkers as Kirk, Spock, Bones and the rest of the gang...