Mudd's Women
McCoy: Well, we found a whole world of minds that work just like yours - logical, unemotional, completely pragmatic - and we poor, irrational humans whipped them in a fair fight. Now you'll find yourself back among us illogical humans again.
Spock: Which I find eminently satisfactory, Doctor, for NOWHERE am I so DESPERATELY needed as among a shipload of illogical humans.

McCoy: Besides, he has avoided two appointments that I've made for his physical exam without reason.
Spock: It's not at all surprising, Doctor. He's probably terrified of your beads and rattles.

Mirror, Mirror: Indeed, gentlemen. May I point out that I had an opportunity to observe your counterparts here quite closely. They were brutal, savage, unprincipled, uncivilized, treacherous - in every way splendid examples of homo sapiens, the very flower of humanity. I found them quite refreshing.

Friday's Child:
Spock: The child was named Leonard James Akaar?
McCoy: Has a kind of a ring to it, don't you think, James?
Captain James T. Kirk: Yes, I think it's a name destined to go down in galactic history, Leonard. What do you think, Spock?
Spock: I think you're both gonna be insufferably pleased with yourselves for at least a month... sir.

A piece of the Action:
Spock: [balking at the prospect of another ride in a car with Kirk at the wheel] Captain, must we?
Capt. Kirk: It's faster than walking.
Spock: But not as safe.
Capt. Kirk: Are you afraid of cars?
Spock: Not at all, Captain. It's your DRIVING that alarms me.

The Changeling - After Kirk argues the Nomad/Tan-Ru probe into self destruction:
Spock: Your logic was impeccable, Captain. We are in grave danger.