It can be from any series, and if you want to pick a favorite from each show go ahead.
Picture very much related.
It’s not necessarily my favorite episode but it pops up always when I think of ST
It’s voyagers episode where they observe the planet which is in its own time pocket, so the whole evolution of the society happens in days from voys perspective
TOS, third season, episode 8: For the World Is Hollow and I Have Touched the Sky
I also love The Cloud Minders from the same season (episode 21), but that first one gets me every time, I have no idea why.
EDITED TO ADD: I also really like most of the SNW episodes, but haven’t seen them enough times to have a clear favorite. Possibly Ad Astra Per Aspera, or Charades, lol.
Voyager 5.17 “Course: oblivion”
Hell of an ending.