No matter what anyone thinks of TOS, or of the merits of "Mirror, Mirror" it has some of the most memorable lines in SciFi TV.
Best lines in this ep:
Tharn: Twelve years, Captain Kirk... or twelve-thousand. We are ethically compelled to deny your demands!
...and you've gotta love Sulu...
Sulu: Any time's a good time!
(too bad Takei was never paired with Loretta Swit)
Kirk: The illogic of waste, Mister Spock. The waste of lives, potential, resources, time. I submit to you that your Empire is illogical because it cannot endure! I submit that you are illogical to be a part of it!
...and just a little later...
Kirk: If change is inevitable, predictable, beneficial, doesn't logic demand that you be part of it?
...and some of the very best in TOS:
Spock: One man cannot summon the future.
Kirk: But one man can change the present. Be Captain of the Enterprise, Mister Spock. Find a logical reason for sparing the Halkans and make it stick. Push till it gives. You can defend yourself better than any man in the fleet.
...and the cherry on top...
Kirk: In every revolution... there's one man with a vision!