A warning for those about to be miffed: I really do like Star Trek, for the most part, watch the current incarnation when possible, and can remember the Original Series when it was originally aired on television. For those who have grown up in the post-Star Wars era, when science fiction is commonly found in theaters and on television, and when special effects are often astounding in quality, trust me when I say that the Original Series can’t be fully appreciated by you. There simply wasn’t any other series of out and out science fiction worth watching (please do not mention that god-awful excuse with the stupid robot and the “evil” doctor).
Nevertheless, from the OP we have:
Almost all Star Trek suffers from the same issues: poor acting, badly written scripts, sets that don’t convey any sense of reality (usually showing advanced cultures looking like refugees from a Flintstone movie). The poor special effects of the Original Series have been replaced by fancy stuff. Sometimes, even the aliens are relatively believable, though I’m not sure why all humanoid aliens other than humans seem to develop weird bony ridges on their heads and necks. But the acting, both by regulars and by guests, is generally very poor.
Really good acting is Gerard Depardieu telling Roxanne that he remembers how she changed her hair one day years ago, does she understand, can she feel his that his soul is finally let free in the night, to make her tremble, trembling he can feel in the strand of jasmine that trails from the balcony. Really good acting is Tom Hanks, without saying a single word for 15 minutes, making you understand the desperation and slow battle in his mind from being cut off from the world on a tiny island. Pretty good acting is William Powell trying hard to make his stand while his family slowly chivvies him into doing what he should, including taking a cab to be baptized. Moderately good acting on a regular basis is Brother Cadfael, or any of a number of characters portrayed during the various books serialized on Masterpiece Theater. In short, it isn’t the sort of acting that includes Deanna Troy groaning about “the pain!!” or Kirk making a member of the High Council of Ardana dig with his bare hands.
So, IMHO, most of what is mentioned so far in this thread consists of anything but good acting. Often, what made us willing to feel moved by what was said or done in Star Trek had to do with our willingness to be submerged in the story, and our needful massive suspension of disbelief, without which we simply would dismiss what we were watching as badly acted, badly scripted junk.
Nevertheless, even into this generalized miasma some pearl drops did fall. Guest actors often provide the best performances, mostly because the regular actors are such poor specimins that they really have to dig deep to come up with something really good (one really has to wonder how Patrick Stewart felt about selling his soul for seven years). One always has to keep in mind that the actors were hamstrung by the generally poor writing; one of the reasons episodes in the Original Series like “City of the Edge of Forever” and “The Trouble With Tribbles” stand out is that they had quality scripts. It’s hard to blame actors for having to try to work with such material. Often, the good performances tend to be the cases of better writing allowing something to shine through; was it the actor or the script that made us cry/laugh?
With this in mind, I’ll say that the following performances certainly managed to convey the characters and or moods for a regular character better than most on the show:
-
Ricardo Montalban as Khan, in both “Space Seed” and “The Wrath of Khan.” Hold up in comparison the far less appealing turn by William Shatner in the movie (really, you cannot mean you consider that stupid scream to be quality acting?).
-
Stanley Adams as Cyrano Jones in “The Trouble With Tribbles.” He managed exactly what the script called for, engaged our emotions, and did so without going quite so far over the top as did Roger Carmel in his two Mudd episodes.
-
William Shatner (surprise!!) as James Kirk in “City on the Edge of Forever.” Not so much because of his overall performance, which was pretty typical, but at least was elevated by the script, as for the scene when Edith Keeler dies, when he actually manages to rise to the occaision. Not great acting, but certainly sufficient to have held its own in any television show.
There are other examples, but I use these to illustrate how I feel on the subject. A really good performance can be the center of attention, or it can be one of the main side pieces, or it can, on occaision, come from the regular characters, usually because they manage to elevate some particular part of an episode. Goodness knows, if it was Trek, and it shined, it required really extra effort! 