Arthur Coleman in Turnabout Intruder was bland at best.
Ed McCready played the doctor on the Exeter like he was from New Jersey. He would have fit in on Janus 6.
Tige Andrews played the Klingon captain in Friday's Child with little gusto. Actually no differently than he did when he was a regular on The Mod Squad. They could almost be the same character.
Julie Newmar was pretty terrible, too. She is such an odd actress anyway. She's great to look at, but she comes off distant in everything.
I know this is heresy, but Marianna Hill played Helen Noel in the most annoying fashion. Maybe it was the character as written, but seriously, she was grating. Everything was an obvious, over the top flirt or push to make Kirk uncomfortable.
I could have lived without Barbara Anderson in Conscience of the King as well.
Seriously, though, the performance that keeps me from watching the episode goes to Jill Ireland. She was beautiful, had a smashing figure (which oddly was not shown on the series known for exploiting women's appearance), but was the worst actress to get a lot of work. No idea why she was so highly employed, she was routinely terrible.