I saw most of TNG and all of Voyager and DS9 during their initial runs as a kid and to this day Voyager is still my favorite Trek series. It can be a little variable at times with guest writers, like the warp 10 thing, and the series finale manages to be one of the most anti-climactic of anything I’ve ever seen, but it had a real killer of an ensemble cast, a larger story, and a surprising amount of character development. And I know I already mentioned the cast but it’s hard not to love mama bear Janeway
My wife has always casually liked TOS but had never seen all of TNG or any of DS9 or VOY. We watched all of them together during lockdown. She liked them all, but Voyager was her favorite of the bunch, too
DS9 Trials and Tribble-ations
it's not an "episode" but Star Trek IV: The Voyage Home
TNG Yesterday's Enterprise I've also always found to be an intriguing glimpse at an era we've never really seen otherwise