I really enjoy "Spock's Brain" and "Requiem for Methuselah" for their music scores and beautiful women.
I kind of enjoy "Whom Gods Destroy" and "The Way to Eden" along those lines as well.
"Assignment: Earth", "The Mark of Gideon", and "The Cloud Minders" are almost good.
But everyone has a different list of clunkers. The lower-tier episodes I'm less fond of, from least bad to most awful, are:
"Friday's Child" - Love-it-or-hate-it music score for the planet-side scenes is too heavy for me; men wearing carpets.
"The Alternative Factor" - Confusing and dumb.
"Plato's Stepchildren" - Embarrassing.
"The Savage Curtain" - Give me a break.
"And the Children Shall Lead" - The worst.