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.