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'd say Nixon but I feel like that's cheating?
I would say the Beastie Boys.
Well considering nixon is voiced by billy west, id say thats cheating. Dude voices almost everybody.