Mirror, Mirror 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.
What other theories should Kirk have considered, that fit the facts as they knew them at that point?
That they are inside some mind trick or under the influence of some malevolent alien? It happened many times before that episode.
They always ran into things that drove the crew mad, delusional or simply degenerate.
The fact that Kirk somehow just guessed the right answer for that situation seemed kind of far-fetched to me.