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.
He also got to do two cameos on 30 Rock, both times he ended the bit by interrupting someone and saying, "Quiet! A whale is in trouble!" and running off.