TNG: Drumhead, I, Borg, and Chain of command (or if you don’t want a two parter, take your pick of Q who, Measure of a Man, or Yesterday’s Enterprise)

DS9: In the Pale Moonlight, Inquisition, and It’s Only a Paper Moon.

VOY: Living Witness, Tuvix, and Year of Hell (or again, if you don’t want a two parter, go for Tinker Tenor Doctor Spy)

TOS: The Devil in the Dark, The City on the Edge of Forever, and Mirror Mirror.

ENT: Similitude, Regeneration, In a Mirror Darkly (don’t really have a good non-two parter to suggeat over In a Mirror, Enterprise doesn’t have a lot of great standalone episodes IMO)

Movies: if you want to use these as examples, The Wrath of Khan for TOS and First Contact for TNG.

And I don’t really have a list for LD, PIC, DIS, SNW, or PRO.