Interesting that no one has mentioned Time Squared. It tackles time travel differently than We'll Always Have Paris, and to my mind it's one of the highlights of Season 2, if not one of the better Trek time travel tales.
It is in the Memory Alpha list, but yes, we haven't discussed it. I was amazed by it when it first aired, and raved about it to friends.
First, it had this unrelenting mood of dread with Picard brooding intensely. And, the ending was rather ambiguous, which was a nice change from most TV shows.
Doug