Well, Sito Jaxa was technically a two-shot character, just like my favorite Trek guest star character, K'Ehleyr, or Tekeny Ghemor or Lon Suder. As for actual one-shots:
TOS
Romulan Commander (Balance of Terror) - the no1 favorite one-shot from TOS
Dr Richard Daystrom (The Ultimate Computer)
Romulan Commander (The Enterprise Incident)
Miranda Jones and Ambassador Kollos (Is There In Truth No Beauty?), though the latter is a borderline case as he was played by Leonard Nimoy and a shiny box, but since the thread is about one-shot characters rather than actors...
Commodore Decker (The Doomsday Machine)
Zarabeth (All Our Yesterdays)
T'Pring (Amok Time) - not that I actually liked her, but she was an interesting character
Gem (The Empath)
Flavius Maximux (Bread and Circuses), but that might have been partly because he was hot
And finally, Horta deserves a mention
TNG
Alidar Jarok (The Defector)
Commander Toreth (The Face of the Enemy) - another great Romulan character
Roga Danar (The Hunted)
Elizabeth Shelby (The Best of Both Worlds)
Ira Graves (The Schizoid Man)
Gul Madred (Chain of Command) - the first complex Cardassian villain in Trek
Jellico (Chain of Command) - like him or not, brought some conflict to Enterprise
Kevin Uxbridge (The Survivors)
DS9
Aamin Marritza (Duet) - if I was to choose one performance of a guest star in Trek as the best, it's Harris Yulin in this episode
Sakonna (The Maquis)
nutty guy from "In the Cards"
Natima Lang (Profit and Loss), partly because I like the way they develop her story in Terok Nor books
Gilora Rejal (Destiny) - hilarious interactions with O'Brien
I'm tempted to say Curzon Dax in Facets, although that's another bortherline case since he was played by Rene Auberjonois...
and technically we did see him in a flashback in The Emissary, but that was for a few seconds and non-speaking so it doesn't count
VOY
the Q who wanted to die (Deathwish)
the Vidiian played by Brian Markinson who stole the face of the crewman played by the same actor (Faces) - the guy was pathetic, creepy and even sort of sympathetic, at the same time
One (Drone)
Kashyk (Counterpoint)
Shannon O'Donnell (11:59) - another character played by a main cast member... Watching it, I realized that I knew her better - what she was like, what made her tick - after one episode than I knew Captain Janeway after 5 seasons. I'm not sure what it says about the VOY writers...
Also, I must be the only person who really liked Tuvix.
On every 'was Janeway right' thread, there's always someone saying how annoying he was. I liked him better than Neelix for sure, and a little less than Tuvok, but I like Tuvok.
I'm sure there are lots of people I've forgotten to mention.
As for ENT... I'm only going through season 2 so it's a bit early to tell yet... so far, Mestral from Carbon Creek was quite likable.
TOS
Romulan Commander (Balance of Terror) - the no1 favorite one-shot from TOS
Dr Richard Daystrom (The Ultimate Computer)
Romulan Commander (The Enterprise Incident)
Miranda Jones and Ambassador Kollos (Is There In Truth No Beauty?), though the latter is a borderline case as he was played by Leonard Nimoy and a shiny box, but since the thread is about one-shot characters rather than actors...

Commodore Decker (The Doomsday Machine)
Zarabeth (All Our Yesterdays)
T'Pring (Amok Time) - not that I actually liked her, but she was an interesting character
Gem (The Empath)
Flavius Maximux (Bread and Circuses), but that might have been partly because he was hot

And finally, Horta deserves a mention

TNG
Alidar Jarok (The Defector)
Commander Toreth (The Face of the Enemy) - another great Romulan character
Roga Danar (The Hunted)
Elizabeth Shelby (The Best of Both Worlds)
Ira Graves (The Schizoid Man)
Gul Madred (Chain of Command) - the first complex Cardassian villain in Trek
Jellico (Chain of Command) - like him or not, brought some conflict to Enterprise
Kevin Uxbridge (The Survivors)
DS9
Aamin Marritza (Duet) - if I was to choose one performance of a guest star in Trek as the best, it's Harris Yulin in this episode
Sakonna (The Maquis)
nutty guy from "In the Cards"
Natima Lang (Profit and Loss), partly because I like the way they develop her story in Terok Nor books
Gilora Rejal (Destiny) - hilarious interactions with O'Brien
I'm tempted to say Curzon Dax in Facets, although that's another bortherline case since he was played by Rene Auberjonois...

VOY
the Q who wanted to die (Deathwish)
the Vidiian played by Brian Markinson who stole the face of the crewman played by the same actor (Faces) - the guy was pathetic, creepy and even sort of sympathetic, at the same time
One (Drone)
Kashyk (Counterpoint)
Shannon O'Donnell (11:59) - another character played by a main cast member... Watching it, I realized that I knew her better - what she was like, what made her tick - after one episode than I knew Captain Janeway after 5 seasons. I'm not sure what it says about the VOY writers...

Also, I must be the only person who really liked Tuvix.

I'm sure there are lots of people I've forgotten to mention.
As for ENT... I'm only going through season 2 so it's a bit early to tell yet... so far, Mestral from Carbon Creek was quite likable.