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.
I liked how they did an entire episode with Beck.
Also, apparently John Goodman was robot santa.
Yes!!! And Mark Hamill was Chanukah Zombie and Coolio was Kwanzaabot