An epp I would not have missed had it not been made:

[Stella's_Funboy]

'The Way to Eden'. Painful.

 

[22 Stars]

The Empath

 

[JiNX-01]

Are we limited to one ep?
'Cause I have a list.

Any episode involving children (I like children but the writers clearly couldn't write for them)

Spock's Brain
The Alternative Factor
Turnabout Intruder

 

[Captain Tracy]

Turnabout Intruder

 

[ALF]

That's the funny thing about Trek... for every outstanding episode there is at least one miserable stinker.

 

[BillJ]

I like Turnabout Intruder.

 

[Michael]

'The Way to Eden'. Painful.

The Empath

REQUIEM FOR METHUSELAZZZZZZZZZZZZZZZzzzzzzzzzzz.....

Wow, some of my favorite episodes right there.

Personally, I could live without "The Omega Glory". Hate that episode, just hate it.

 

[NTRPRZ]

I agree. For each gem there's a stinker, and there are too many stinkers to pick just one. If I were forced to name one, it'd be "Plato's Stepchildren." The sight of Kirk on all fours doing a lousy imitation of Secretariat was THE low point of the series.

"Lost in Space" may have been renown for crud like "The Great Vegetable Rebellion," where Smith was turned into a stalk of celery, but at least they didn't put the star of the show -- Guy Williams -- through the embarrassment of pretending to be a horse with a little person as a jockey (something tells me Williams would have punched Irwin Allen in the nose if he'd even suggested it).

Seeing Uhura in something other than her Starfleet miniskirt was the only part of "Plato" worth watching.

 

[BillJ]

'The Way to Eden'. Painful.

The Empath

REQUIEM FOR METHUSELAZZZZZZZZZZZZZZZzzzzzzzzzzz.....

Wow, some of my favorite episodes right there.

Personally, I could live without "The Omega Glory". Hate that episode, just hate it.

The Omega Glory is in my TOS Top 5.

 

[RAMA]

During one of the TOS rewatch threads I thought about Turnabout Intruder as the worst episode, it really probably is much worse than Spock's Brain. So that is my answer.

RAMA

 

[Admiral Buzzkill]

So many.

The Nazi Planet is the first that springs to mind.

 

[AtoZ]

Plato's Stepchildren. An outright embarrassment to the show and to me as a fan. It's Trek that I feel as though I need to apologize for.

 

[smalltalk66]

The Apple
Plato's Stepchlidren
Any Trek episode about Nazis

 

[trekkiebaggio]

And the Children Shall Lead

 

[RandyS]

City on the Edge of Forever.

William Shatner and Joan Collins mooning over each other for an hour? Forty one years on, I still don't get it.

 

[Harvey]

"The Alternative Factor." It's a blemish on an otherwise consistently good first season.

 

[T'Bonz]

Plato's Stepchildren. Makes me cringe.

And the Children Shall Lead is second.

 

[Maurice]

I'm there with Planet Nazi.

 

[Rulius]

Alternative factor. Worst episode of TOS and possibly all of Trekdom.

 

[Delta Vega]

A Piece of the Action
Platos Stepchildren
The Apple
Patterns of Force
Spocks Brain


Off the top of my head