I'm glad to see Mark Lenard won this little poll (RIP). Well deserved, although that doesn't belittle the other guest stars, many of whom were terrific in their own right.
For my money, one of the best (and most understated) performances in all of TOS was Logan Ransey as the sly, oily Roman proconsul Claudius in "Bread and Circuses." If the episode works for you at all, it's because of him. He was always so good in any TV program I've seen him.
Many actors . . . you can tell they're acting. Ramsey made it appear effortless. It's a shame he's not better remembered.
I'm glad you mentioned him. I feel the same way about his performance. One of the best, a true television thespian.