I would say that “Year of Hell” or “Macrocosm” would be a great introduction into Voyager. I, personally, love Nemesis, but if you aren’t already familiar with the Voyager scenario and crew, seeing Chakotay in that situation and then juxtaposed against what is occurring on the ship won’t have the same impact if someone is new to Trek.

“Year of Hell” is not only a great Voyager episode(s), but it is fantastic television. It would leave a newcomer with “Wow” factor that would make them want even more. Same with “Macrocosm”; these episodes put the captain at the center and really allow a new viewer to appreciate Janeway’s character.

For DS9, duet is great, but I think focusing on the effects of the Cardassian occupation of the Bajoran people is the strongest element of the show which is where a Garak-centered episode would shine best. Starting with either “Past Prologue” or “Cardassians” would help introduce DS9 while also giving focus to one its most brilliant characters.

TNG is much harder because the best episodes of TNG rely on having gone on some of the earlier journeys with them. I would say that “Symbiosis” is very classic Trek that would make someone eager to see more of the crew and more of the available stories. If the newcomer has any understanding of TOS-era Trek, then I would add “A Matter of Honor” because we get so much Klingon culture while still experiencing the whole crew.

These episodes are all a mix of the psychological, but also more action and character-based elements of Trek that make 90s Trek so beloved. Also, don’t discount Lower Decks or Picard because they are newer, but do save those for after you’ve already had them experience all of TNG, DS9, and Voyager because those are all necessary to appreciate the latter two.