This list is probably the most fun to put together: my tribute to the biodiversity of Trek! A list of introductory appearances of the aliens that went on to become recurring or ongoing presences in the Trekverse (some of them much more prominently than others -- some of these we only see once, but hear more about beyond that appearance; I was basically looking for as many aliens as I could find that resonated beyond the confines of a single episode).
THE RULES. The basic idea is your standard elimination game. Below is a list of episodes and movies that introduce an array of different Star Trek species. Eliminate your FAVORITE or the one you deem the 'BEST.' Please provide an EXPLANATION for why you are eliminating your choice and be sure to copy and paste the list with your choice removed.
Leave at least 2 eliminations by other posters before you eliminate another episode.
A NOTE ON ELIMINATION CRITERIA: Feel free to eliminate based on your love of the aliens, the episode, or how the episode introduces the aliens -- technically we're eliminating the episode, but there's no "wrong" answer for how to frame what makes that episode your favorite, it can be just about the listed aliens.
No tactical voting! You cannot remove an episode just because you feel it would threaten your preferred episode's chance to win.
In the 2019 Least Disliked games, this was run as "The Least Disliked Introduction Of Recurring Alien Antagonists" -- so it excluded all the friendly or ally species that are on this iteration of the list -- and the winner was the ROMULANS: "Balance Of Terror", TOS season 1.
Finally, a big thanks to @Farscape One for helping to put this list together!
Enjoy!
VULCANS, ORIONS, & TALOSIANS: “The Cage”, TOS Season 1
ANDROIDS/SYNTHETICS: “What Are Little Girls Made Of?”, TOS Season 1
ROMULANS: “Balance Of Terror”, TOS Season 1
GORN: “Arena”, TOS Season 1
COMPUTER AI’S RUN AMOK: “The Return Of The Archons”, TOS Season 1
AUGMENTS: “Space Seed”, TOS Season 1
HORTA: “The Devil In The Dark”, TOS Season 1
KLINGONS & ORGANIANS: “Errand Of Mercy”, TOS Season 1
MIRROR UNIVERSE TERRANS: “Mirror, Mirror”, TOS Season 2
TRIBBLES: “The Trouble With Tribbles”, TOS Season 2
ENORMOUS SPACE DWELLING ORGANISMS: "The Immunity Syndrome", TOS Season 2
ANDORIANS & TELLARITES: “Journey To Babel”, TOS Season 2
THOLIANS: “The Tholian Web”, TOS Season 3
EDOSIANS: “Beyond The Farthest Star”, TAS Season 1
CAITIANS: “The Survivor”, TAS Season 1
THE KZINTI: “The Slaver Weapon”, TAS Season 1
DELTANS & SAURIANS: “Star Trek: The Motion Picture”, TOS Feature
EFROSIANS: “Star Trek IV: The Voyager Home”, TOS Feature
BETAZOIDS & THE Q: “Encounter At Farpoint”, TNG Season 1
FERENGI: “The Last Outpost”, TNG Season 1
THE TRAVELER'S SPECIES: "Where No One Has Gone Before", TNG Season 1
THE ANTICANS & THE SELAY: “Lonely Among Us”, TNG Season 1
MR. HOMN’S SPECIES: “Haven”, TNG Season 1
HOLOGRAMS THAT BECOME SELF-AWARE/REAL/HAVE THEIR SAFETIES TURNED OFF: “The Big Goodbye”, TNG Season 1
BYNARS: “11001001”, TNG Season 1
BENZITES: “Coming Of Age”, TNG Season 1
BOLIANS & THE ALIEN PARASITES: “Conspiracy”, TNG Season 1
EL-AURIANS: “The Child”, TNG Season 2
BORG: “Q Who”, TNG Season 2
PAKLEDS: “Samaritan Snare”, TNG Season 2
ZAKDORN: “Peak Performance”, TNG Season 2
BARZAN: “The Price”, TNG Season 3
RISIANS: “Captain’s Holiday”, TNG Season 3
CARDASSIANS: “The Wounded”, TNG Season 4
TRILL: “The Host”, TNG Season 4
BAJORANS: “Ensign Ro”, TNG Season 5
KTARIANS: “The Game”, TNG Seson 5
CORVALLENS: "Face Of The Enemy", TNG Season 6
NAUSSICANS: “Tapestry”, TNG Season 6
YRIDIANS: “Birthright, Part 1”, TNG Season 6
THE SON’A: “Star Trek: Insurrection”, TNG Feature
REMANS: “Star Trek Nemesis”, TNG Feature
THE PROPHETS, MARKALIANS, & LURIANS: “Emissary”, DS9 Season 1
BOSLICS: “Babel”, DS9 Season 1
HUPYRIANS: “The Nagus”, DS9 Season 1
DOPTERIANS: "The Forsaken", DS9 Season 1
KOBHEERIANS: "Duet", DS9 Season 1
JEM’HADAR & VORTA: “The Jem’Hadar”, DS9 Season 2
THE FOUNDERS & THE KAREMMA: “The Search, Part 1”, DS9 Season 3
LETHEANS: “Distant Voices”, DS9 Season 3
BREEN: “Indiscretion”, DS9 Season 4
THE PAH-WRAITHS: “The Assignment”, DS9 Season 5
KAZON, TALAXIANS, OCAMPANS, & THE CARETAKER'S SPECIES: “Caretaker”, VOY Season 1
VIDIIANS: “Phage”, VOY Season 1
SPECIES 8472: “Scorpion”, VOY Season 3
KRENIM: “Year Of Hell”, VOY Season 4
HIROGEN: “Message In A Bottle”, VOY Season 4
MALON: “Night”, VOY Season 5
THE OVERLOOKERS: “Tinker Tenor Doctor Spy”, VOY Season 6
VAADWAUR: “Dragon’s Teeth”, VOY Season 6
BRUNALI, WYSANTI, & NORCADIANS: “Collective”, VOY Season 6
SULIBAN & DENOBULANS: “Broken Bow”, ENT Season 1
KREETASANS: “Vox Sola”, ENT Season 1
THE XINDI: “The Xindi”, ENT Season 3
THE SPHERE BUILDERS: “Harbinger”, ENT Season 3
KEENSER’S SPECIES: “Star Trek”, Kelvin Feature
KELPIENS: “The Vulcan Hello”, DIS Season 1
TARDIGRADES: "Context Is For Kings", DIS Season 1
THE JAHSEPP: “An Obol For Charon”, DIS Season 2
THE BA’UL: “The Sound Of Thunder”, DIS Season 2
XAHEANS: “Runaway”, ST Season 1
THE RULES. The basic idea is your standard elimination game. Below is a list of episodes and movies that introduce an array of different Star Trek species. Eliminate your FAVORITE or the one you deem the 'BEST.' Please provide an EXPLANATION for why you are eliminating your choice and be sure to copy and paste the list with your choice removed.
Leave at least 2 eliminations by other posters before you eliminate another episode.
A NOTE ON ELIMINATION CRITERIA: Feel free to eliminate based on your love of the aliens, the episode, or how the episode introduces the aliens -- technically we're eliminating the episode, but there's no "wrong" answer for how to frame what makes that episode your favorite, it can be just about the listed aliens.
No tactical voting! You cannot remove an episode just because you feel it would threaten your preferred episode's chance to win.
In the 2019 Least Disliked games, this was run as "The Least Disliked Introduction Of Recurring Alien Antagonists" -- so it excluded all the friendly or ally species that are on this iteration of the list -- and the winner was the ROMULANS: "Balance Of Terror", TOS season 1.
Finally, a big thanks to @Farscape One for helping to put this list together!
Enjoy!

VULCANS, ORIONS, & TALOSIANS: “The Cage”, TOS Season 1
ANDROIDS/SYNTHETICS: “What Are Little Girls Made Of?”, TOS Season 1
ROMULANS: “Balance Of Terror”, TOS Season 1
GORN: “Arena”, TOS Season 1
COMPUTER AI’S RUN AMOK: “The Return Of The Archons”, TOS Season 1
AUGMENTS: “Space Seed”, TOS Season 1
HORTA: “The Devil In The Dark”, TOS Season 1
KLINGONS & ORGANIANS: “Errand Of Mercy”, TOS Season 1
MIRROR UNIVERSE TERRANS: “Mirror, Mirror”, TOS Season 2
TRIBBLES: “The Trouble With Tribbles”, TOS Season 2
ENORMOUS SPACE DWELLING ORGANISMS: "The Immunity Syndrome", TOS Season 2
ANDORIANS & TELLARITES: “Journey To Babel”, TOS Season 2
THOLIANS: “The Tholian Web”, TOS Season 3
EDOSIANS: “Beyond The Farthest Star”, TAS Season 1
CAITIANS: “The Survivor”, TAS Season 1
THE KZINTI: “The Slaver Weapon”, TAS Season 1
DELTANS & SAURIANS: “Star Trek: The Motion Picture”, TOS Feature
EFROSIANS: “Star Trek IV: The Voyager Home”, TOS Feature
BETAZOIDS & THE Q: “Encounter At Farpoint”, TNG Season 1
FERENGI: “The Last Outpost”, TNG Season 1
THE TRAVELER'S SPECIES: "Where No One Has Gone Before", TNG Season 1
THE ANTICANS & THE SELAY: “Lonely Among Us”, TNG Season 1
MR. HOMN’S SPECIES: “Haven”, TNG Season 1
HOLOGRAMS THAT BECOME SELF-AWARE/REAL/HAVE THEIR SAFETIES TURNED OFF: “The Big Goodbye”, TNG Season 1
BYNARS: “11001001”, TNG Season 1
BENZITES: “Coming Of Age”, TNG Season 1
BOLIANS & THE ALIEN PARASITES: “Conspiracy”, TNG Season 1
EL-AURIANS: “The Child”, TNG Season 2
BORG: “Q Who”, TNG Season 2
PAKLEDS: “Samaritan Snare”, TNG Season 2
ZAKDORN: “Peak Performance”, TNG Season 2
BARZAN: “The Price”, TNG Season 3
RISIANS: “Captain’s Holiday”, TNG Season 3
CARDASSIANS: “The Wounded”, TNG Season 4
TRILL: “The Host”, TNG Season 4
BAJORANS: “Ensign Ro”, TNG Season 5
KTARIANS: “The Game”, TNG Seson 5
CORVALLENS: "Face Of The Enemy", TNG Season 6
NAUSSICANS: “Tapestry”, TNG Season 6
YRIDIANS: “Birthright, Part 1”, TNG Season 6
THE SON’A: “Star Trek: Insurrection”, TNG Feature
REMANS: “Star Trek Nemesis”, TNG Feature
THE PROPHETS, MARKALIANS, & LURIANS: “Emissary”, DS9 Season 1
BOSLICS: “Babel”, DS9 Season 1
HUPYRIANS: “The Nagus”, DS9 Season 1
DOPTERIANS: "The Forsaken", DS9 Season 1
KOBHEERIANS: "Duet", DS9 Season 1
JEM’HADAR & VORTA: “The Jem’Hadar”, DS9 Season 2
THE FOUNDERS & THE KAREMMA: “The Search, Part 1”, DS9 Season 3
LETHEANS: “Distant Voices”, DS9 Season 3
BREEN: “Indiscretion”, DS9 Season 4
THE PAH-WRAITHS: “The Assignment”, DS9 Season 5
KAZON, TALAXIANS, OCAMPANS, & THE CARETAKER'S SPECIES: “Caretaker”, VOY Season 1
VIDIIANS: “Phage”, VOY Season 1
SPECIES 8472: “Scorpion”, VOY Season 3
KRENIM: “Year Of Hell”, VOY Season 4
HIROGEN: “Message In A Bottle”, VOY Season 4
MALON: “Night”, VOY Season 5
THE OVERLOOKERS: “Tinker Tenor Doctor Spy”, VOY Season 6
VAADWAUR: “Dragon’s Teeth”, VOY Season 6
BRUNALI, WYSANTI, & NORCADIANS: “Collective”, VOY Season 6
SULIBAN & DENOBULANS: “Broken Bow”, ENT Season 1
KREETASANS: “Vox Sola”, ENT Season 1
THE XINDI: “The Xindi”, ENT Season 3
THE SPHERE BUILDERS: “Harbinger”, ENT Season 3
KEENSER’S SPECIES: “Star Trek”, Kelvin Feature
KELPIENS: “The Vulcan Hello”, DIS Season 1
TARDIGRADES: "Context Is For Kings", DIS Season 1
THE JAHSEPP: “An Obol For Charon”, DIS Season 2
THE BA’UL: “The Sound Of Thunder”, DIS Season 2
XAHEANS: “Runaway”, ST Season 1
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