In a Mirror Darkly
does it for me.
comes as a close second, but my issue with it is how quickly Kirk formulates the theory of being in a parallel universe. Those lines kind of felt like artificially put there to make the watcher understand exactly what is happening.
was pretty good too. The first time I watched that episode in the 90's, I was all over it, as I also remembered the Kirk incident.
I admit, I did not watch the rest of DS9 episodes (I never watched past season 3), so I have no opinions on them.