While there are so many actors and pop-culture icons who added to the beauty of this show, being a huge Star Trek geek, I have to go with the cast that participated in the episode Where No Fan Has Gone Before
Honorable mention to Bea Arthur as Femputer.
Bea Arthur. The loaded way she says "Why?" Is incredible. They even call it out on the DVD commentary.
I DID NOT REALIZE... ALLL HAIL THE MIGHTY FEMPUTER!!!!!