I'll save The Trill. The Host itself isn't the best episode, but I just loved the concept of the race, and was happy they were expanded on in DS9.
ENORMOUS SPACE DWELLING ORGANISMS: "The Immunity Syndrome", TOS Season 2
EDOSIANS: “Beyond The Farthest Star”, 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 Voyage Home”, TOS Feature
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
BENZITES: “Coming Of Age”, TNG Season 1
BOLIANS & THE ALIEN PARASITES: “Conspiracy”, TNG Season 1
PAKLEDS: “Samaritan Snare”, TNG Season 2
BARZAN: “The Price”, TNG Season 3
CORVALLENS: "Face Of The Enemy", TNG Season 6
YRIDIANS: “Birthright, Part 1”, TNG Season 6
THE SON’A: “Star Trek: Insurrection”, TNG Feature
REMANS: “Star Trek Nemesis”, TNG Feature
BOSLICS: “Babel”, DS9 Season 1
HUPYRIANS: “The Nagus”, DS9 Season 1
DOPTERIANS: "The Forsaken", DS9 Season 1
KOBHEERIANS: "Duet", DS9 Season 1
THE PAH-WRAITHS: “The Assignment”, DS9 Season 5
MALON: “Night”, VOY Season 5
BRUNALI, WYSANTI, & NORCADIANS: “Collective”, VOY Season 6
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