10 Best Alternate Reality Episodes In TV Shows

[ChrisIII]

Doctor Who's "Inferno". Also has the added twist of the Doctor trying to prevent a catastrophe that happens on the alternate world.

[TheBlackTerror]

Not exactly the exact question the OP asked but the terrific TV movie adaptation of LeGuin’s The Lathe of Heaven essentially has multiple alternate realities

[Jokerz79]

Just dawned on me Mirror Mirror and all the Mirror Universe Episodes are good especially ENT In the Mirror Darkly but the best Star Trek episode to deal with an alternate world/timeline was Yesterday Enterprise.

[K7P5V]

Since animation is allowed, my choice would be "Future Tense" from Gargoyles

[CaptCleghorn]

Just dawned on me Mirror Mirror and all the Mirror Universe Episodes are good especially ENT In the Mirror Darkly but the best Star Trek episode to deal with an alternate world/timeline was Yesterday Enterprise.

I loved Yesterday's Enterprise, but it was too big for me. I much prefer Parallels as it focused on small details. Both of these episodes did have lasting ramifications, though with Sela's birth and Worf/Troi.

[Catlady in training]

Doctor Who: Turn Left

[ChrisIII]

I just saw the Superman Animated series episode Brave New Metropolis, kind of interesting how it predates Injustice, the Knightmare stuff from the Snyderverse (and to a lesser extent Kingdom Come and Infinite Crisis) by having Superman go evil or badly disillusioned because of Lois's death. Although admittedly my Superman knowledge is a bit rusty.