"Requiem for Methuselah" is the unpopular (?) or at least often-criticized episode that I like the most.
"Spock's Brain" is the least-popular episode that I like unreservedly.
I'm also a big fan of "The Lights of Zetar" and "All Our Yesterdays."
I'm willing to suffer through "Plato's Stepchildren" for that one wonderful moment at the very end.
I can take "The Way to Eden" because I secretly like the songs.
I go back and forth on "The Alternative Factor," but I never think it's very good.
"And the Children Shall Lead" and "The Savage Curtain" are the only two that I really don't like, and even they have their moments.