Quite a number of performers haven't been mentioned yet - some of whom I deposit below.
The strengths/characteristics of TOS were that this series was theatrical and literate. I select "Who Mourns For Adonais?" as the episode with a most affecting guest cast (both Leslie Parrish as well as Michael Forest). Parrish transitioned from professional technician to defensive objector to hero worship to spurning flirt and to sobbing rape victim.
-) Alfred Ryder in "The Man Trap" -) John Abbott and John Colicos in "Errand Of Mercy" -) Nancy Kovack in "A Private Little War" -) Jay Robinson in "Elaan Of Troyius" -) Michael Dunn in "Plato's Stepchildren" -) Frank Gorshin in "Let That Be Your Last Battlefield" -) James Daly in "Requiem For Methuselah" -) Sandra Smith in "Turnabout Intruder"
I consider Sharon Acker from "The Mark Of Gideon" to be the most attractive female guest star, IMO.
Special mention for Marj Dusay from "Spock's Brain" who went from bimbo to straight "A"s in a matter of moments.
I think John Colicos' sly charm as Klingon "Military Governor" of Organia Kor in Errand of Mercy is an underappreciated gem. And writer Gene Coon knocked what could have been an average French Undergroud vs. the Gestapo-type pastiche out of the park.
