I've noticed an abundance of episodes that used the word 'fool' as an insult in Star Trek. It seems to be the Go-To Word to insult everyone from Kirk, to Captain Tracy to Spock.
Here's a list of what I found so far...

'Where No Man Has Gone Before':
Mitchell to Dehner about Lee; "He's a fool...a fool. He'd seen those points and hadn't noticed their condition!"
Mitchell in the transporter room; "You fools, soon I'll squash you like insects!"
Mitchell to Kirk on Delta Vega; "You should have killed me when you could have, James. Command and compassion is a fool's mixture."

'Miri':
Not an exact quote, but the kid's games are called 'Fooley's.

'The Menagerie':
Vina's response to Pike questioning; "You're a fool!" Pike - "And since you're not real there's not much point in continuing this conversation, is there?"

'Errand of Mercy':
Kor becomes fristrated with the Organians; "Fools! Will I have to kill them all?!"

'The Alternative Factor';
Lazarus becomes frustrated with Kirk when he doesn't give him some dilithium crystals to hunt down his other self; "You fool! There won't be any ship if we don't kill him! He'll destroy all of you!"

'City On The Edge Of Forever';
Spock realizes he's simply been watching history replayed on the Guardian; "I am a fool. My tricorder is capable of recording even at this speed-- I've missed taping centuries that no living man..."

'Catspaw';
Kirk seduces Sylvia and she's ready to switch sides; Kirk - "What about Korob?" Sylvia - "He is a fool. I'll do without him."
Later, Kirk rejects an offer to join Sylvia; "You fool! Don't you realize what you're giving up?! Everything your species finds desireable!"

'Friday's Child':
Again not a quote, but Scotty quotes the old saying, "Fool me once, shame on you. Fool me twice, shame on me."

'The Omega Glory':
Twice Kirk can't believe that Captain Tracy has broken the Prime Directive;
Spock - "Captain Tracy's reserve belt packs...empty. Found among the remains of several hundred Yang bodies." Kirk - "The fool!"
Spock - "However you must, at the very least, confiscate his phaser." Kirk - "The fool! Starfleet should be made aware..."

'Bread And Circuses':
Maximus doubts his religion; "The message of the Son that all men are brothers was kept from us. Perhaps I'm a fool to believe it."

'And The Children Shall Lead'
Scotty notices that the ship is out of orbit, despite what one of his (controlled) engineers thinks.
"Ya blind fool! Can't ya see what's in front of ya?!"
Kirk confronts the Gorgan; "...Not if we join together to fight you!" Gorgan - "Foolish! You will be destroyed!"

'Day of the Dove':
An entity-controlled McCoy insults Kirk and Spock on the bridge; "How many more men must die before you two are going to start acting like military men?! Instead of fools?!"
Kang rejects the entity's control; "Only a fool fights in a burning house! Ooout!"

'Requiem For Methuselah':
Flint insults Kirk; "How childish he is, Reena. Would you call him brace...or a fool?"

'The Way To Eden':
Severin leaps from the safety of the shuttlecraft to grab a piece of fruit on a tree; McCoy - "Back here, you fool!" Kirk - "Severin, don't! You'll kill yourself!"

'All Our Yesterdays':
McCoy collapses in the blizzard, but Spock wants him to continue; "Don't be a fool! I can't feel my feet, my hands and face are frostbitten!"

Well, it was the 1960s. You couldn't exactly call someone a douche nozzle or an asshat.

What about 'assclown'? Oh, yeah, Chris Jericho wasn't famous yet!
I have two more, both from 'Tomorrow Is Yesterday'.
Kirk tells Christopher that if he's duty-bound to report, there won't be any evidence to back him up, to which Captain Christopher replies, "That makes me out to be either a fool or a liar."
Later, as Christopher holds Kirk and Sulu at gunpoint and refuses to return to the Enterprise, Kirk warns him, "Don't be a fool! You know what's at stake!"

And another from 'Requiem for Methuselah', when Flint reveals his true identity...
"I was Akharin. A soldier, a bully. And a fool. I fell in battle, pierced through the heart...and did not die."

And two more...
The Naked Time; When Sulu, infected by the disease, tries to convince Riley to join him down in the gym and sneaks off the bridge behind Spock's back, Riley hisses under his breath, "Sulu, don't be a fool! Sulu!"

The Changeling; After Kirk admits that he's a biological unit to Nomad, Spock tells him it wasn't wise, and Kirk agrees, saying, "It was a foolish mistake."

Some more...

"Day of the Dove" - As Spock and Scott have their arguement on the bridge, Spock admits to working with humans has been difficult. "I find their illogic and foolish emotions a constant irritant."

And later in the same episode, as Kirk and Kang have their sword fight, Kang growls,
"No doubt you will reassemble, after I've hacked you to bits!", to which his wife mutters off to one side, "You fool!"

"The Gamesters of Triskelion" - Kirk, Chekov, and Uhura try to run away from their cells for the first time, only to be choked out by their collars of obedience. Galt approaches Kirk, and tells him, "That was foolish, Captain. Escape is quite impossible."

Still more foolish stuff...

Who Mourns From Adonais? - Rebuffed by Kirk, Apollo complains to Carolyn, "Fools! I offer them more than they can know!"'

"For The World Is Hollow And I Have Touched The Sky' - The Oracle has killed the Old Man, and Natira delivers a type of eulogy about him, saying, "--for he was an old man, and old men are sometimes foolish."

I think these are the last ones I could find...
"The Mark of Gideon " - The bridge crew has endured a frustrating conversation with Ambassor Hodin, and McCoy's response to Spock is, "That doesn't mean we have to behave like children and listen to some fool lecture by a diplomat."

"That Which Survives" - Scotty's magnetic probe isn't working, and he insists that Spock jettison him from the warp containment compartment. "Don't be a fool! Push the button! I'm gonna die anyway!"

"The Squire of Gothos" - Kirk points out that Trelane isn't very smart, by telling him, "Because you're a bumbling, inept, fool."