Cheers, stewie-3.

Turnabout wasn't a favorite of mine at all, but each to their own, and that.
Besides, how could I disagree with someone with your handle?

For worst performance by a regularI'm gonna go with Kirk's performance in "Turnabout Intruder."
I actually think he's terrific in that one; especially in the courtroom scenes ("Silence! You will be silent!"). For me, the one performance in that episode that really falls flat is from Sandra Smith. As Janice Lester she doesn't seem to do a thing to try and convey that Kirk's personality is occupying her body, and if any actor had mannerisms and inflections that could've been imitated, it's William freaking Shatner. Really uninspired performance.

Don't eat anything before you watch it.

I have no recollection of that episode!!
I must have accidentally missed or something I will return when watched.
Those are my principles, and if you don't like them well, I have others.

For guest stars, I have to agree with the pick of Melvin Belli. Granted, he was a lawyer, not an actor (dammit, Jim!), but it was just a dreadful showing.
For worst performance by a regularI'm gonna go with Kirk's performance in "Turnabout Intruder." I get that Janice Lester is supposed to be unstable and all that, but Shatner is pretty over the top, even at that.

The real answer here is, was, and will always be, Melvin Belli.
You beat me to it. Even the kids in that episode turned in better performances.

Robert Brown as Lazarus in The Alternative Factor stands out as one of the most memorable bad acting performances to me.
You're right, but in his defense, Brown was a last-moment replacement for an actor who breached his contract. Brown was much better seen in the fondly remembered sit-com
Here Come the Brides
.
Which performance stands out as the worst of the many campy, scene-chewing and simply bad performances in the series?
The real answer here is, was, and will always be, Melvin Belli.

I actually struggle a bit with Nichols's performance. She is alright but her reactions and background actions always seemed a bit off. Not a whole lot but just enough to sometimes dislike her performance.
Interesting Point. I liked her in every scene she did. Care to elaborate. Just curious.

I think Doohan and Nimoy stands out as the ones who managed to convey familarity with technical phrases of the future best while also providing some good performances.
Definetly Nimoy was perfectly cast.
Takei I am just very fond of no matter what he did.
Shatner was a bit off and on in that when he was serious he did really well but pushing the psyche of Kirk he would mostly go a bit overboard. But such was the times I think. However with Shatner you just grow to like that about him. So that is redeeming I think.
I actually struggle a bit with Nichols's performance. She is alright but her reactions and background actions always seemed a bit off. Not a whole lot but just enough to sometimes dislike her performance.
Those are my principles, and if you don't like them well, I have others.

Marlon Brando couldn't have done better.

Earthhhhhling. I will be mercccciful and quick.

THAT was the equation.

Sounded an awful lot like.
Balok
.

The voice made up for it.

That Gorn in the Enterprise episode A Mirror Darkly was quite lethal and agile.

But, but, now his eyes blink. Not so wooden as we were led to believe.

What about the Gorn? The most wooden acting and no expression on his face at all!
Luke Skywalker, your Mom was hawt!
Darth Vader

I'd Rate the cast as follows:
Best actors
Nichelle Nichols
DeForest Kelly
James Doohan
All these actors were able to handle anything thrown at them. Comedy, drama, whatever. Nichols was criminally underused.
Good with what they Gave Him
George Takei - except for some good acting as a no-goodnick in "mirror mirror" and a stint as a swordsman, Takei was given much to do except say "Ahead warp factor 2".
Pretty Limited
A fan of the young girls, Walter Koenig was pretty bad when he was forced to be anything other than charming likable, or express some basic emotion.
Good when he didn't Ham..itup
When Shatner himself, or a director, kept him in check he was pretty good. Sometimes he bit off more than he could chew or was almost laughably over the top. But you can't say he was a wooden or inexpressive.
The most perfectly cast actor
Nimoy - its impossible to imagine anyone else in the role.

It seems the only people who thought he could act were himself, and the TOS producer who hired him.

Nimoy really nailed how it should be done, and Mark Lenard followed suit with an excellent performance.
Yes, once he realized how to play Spock (for instance, no more smiling like in "Where No Man Has Gone Before") Nimoy was just excellent in that role. And I thought Barry Atwater did a great job in "The Savage Curtain", too.

I think that is exactly why they made Spock half human, as well as explaining his knowledge of Terran history.
Nimoy of course also had the luxury of being the first person ever to portray a Vulcan making his performance whether good or bad the only guide there is.
Changes in narrative, dialogue and acting methods over time also make it hard to match the original series. So it is a tough job to follow Nimoy who did it perfectly.
Mark Lenard was also perfect for the role.
Those are my principles, and if you don't like them well, I have others.

And yet T'Pol somehow seemed like she was always on the edge of an emotional breakdown. Not that I think Blalock is fully to blame for that.
It is true that playing a full Vulcan doesn't give you much opportunity to show great range, but Nimoy really nailed how it should be done, and Mark Lenard followed suit with an excellent performance.

Not a fair comparison because Spock is half human and the others are all Vulcan.