Not an episode, but given how many people dislike The Motion Picture I usually end up defending it as it is one of my favorite pieces of Trek.
I only rewatched it a year or so ago, for the first time since TNG was around. I just remember it being so bad it wasn't worth a rewatch.
Time... has improved it somewhat. While the effects are still needlessly drawn out and don't serve any purpose, the dialogue is actually hilarious in retrospect with the sexual puns. Spock announcing they've achieved full penetration as the Enterprise flies into a suggestively shaped hole, things like that.
Not enough to move it from it's position at the bottom half of the movies list, but enough to make it bearable to watch again!