Diana Muldaur - most famous for the season long appearance as Dr. Pulaski on TNG - was a main guest star on two different TOS episodes (as two different characters, Anne Mulhall in Return to Tomorrow and Miranda Jones in Is There in Truth No Beauty).
All three characters were memorable in different ways, though I've always thought Pulaski deserved more development than she got as a major character. But Lt. Cmdr. Mulhall in particular is one of my favorite guest characters in the franchise. The chemistry she had with Kirk was incredible and I wish we lived in an alternate universe where Muldaur was allowed to join the main cast on a weekly basis as Anne Mulhall. She would've brought a different and fantastic dynamic to the bridge.
All three characters were memorable in different ways, though I've always thought Pulaski deserved more development than she got as a major character. But Lt. Cmdr. Mulhall in particular is one of my favorite guest characters in the franchise. The chemistry she had with Kirk was incredible and I wish we lived in an alternate universe where Muldaur was allowed to join the main cast on a weekly basis as Anne Mulhall. She would've brought a different and fantastic dynamic to the bridge.