This is a toss up for me between City on the Edge of Forever and Star Trek IV The Voyage Home. I’m siding with City on the Edge of Forever because it is simply an exceptional episode that expanded the scope of an emotional story that could be told within an hour long TOS episode.
I really wish that they revisited the Guardian of Forever in TNG, it doesn’t feel like a massive jump to have this dormant Guardian wake up during this era. Maybe demand to speak to Kirk and they send for his equivalent in Picard. Just feels like a great concept to leave on the shelf.
This is part of the 30 Day Star Trek Challenge – check out the details here.