Gaines blackmails Jack into putting an earpiece on:
Gaines: Now say something.
Jack: If you hurt my daughter, I will kill you.
Gaines: Good, that's a nice clear signal.
Day 2: Jack needs to get access to a criminal organization's inner circle quickly, and the only way he can think of is to bring them one of their enemies. His superiors reluctantly give him access to one of the worst criminals they have access to — only for Jack to shoot him dead the minute he sees him as there is no time to convince the criminal to play along, and Jack believes there is a quicker way to gain the organization's trust:
At the end of Day 2, after spending the whole day pissing on everyone's chips, outright supporting the warmongering in Washington, arresting Tony and Michelle and generally being an arse, Ryan Chappelle is sitting at Tony's desk after Jack, Tony and Michelle's efforts have saved the day:
Chappelle: Tony, that was District Director Vaughn. He's so happy with what we did here today, he doesn't know where to start.
Tony: That's great.
Chappelle: So... what's up, my friend?
Tony: Well, uhh... it's like this. Either fire me, or get out of my chair.
In the first episode of Season 3, Adam believes that Kim shouldn't be working in CTU and thinks she's only there because of her father. Kim's response to this is nonchalantly locking him out of the system and only lets him in again once he takes back his words.
Season 3, pretty much the entirety of Tony's interaction with Nina in the interrogation room, starting with Nina's attempted Hannibal Lecture, climaxing at the revelation that her pulse notably spiked when Tony told her that the medical records of the man they were looking for, who she had claimed she'd never actually met in person, showed that he had been treated for HIV, and ending with her calmly answering one of Tony's questions by informing him that his neck has started bleeding again.
Season 3, episode 23: Sherry getting punched in the face. Not quite as funny as Mandy, but she sure had it coming for a long time.
Season 4, Jack and Audrey have to lay low for awhile at Tony's place, and they can't have anyone who knows that they're there leave the house, so Tony's new girlfriend gets detained when she has to go to work. Jack's usual methods of dealing with people aren't working so well, so Audrey tries to talk things out, girl to girl.
Audrey: Your boss will have to understand.
Jen: And what if he doesn't?
Audrey: I could have him killed.
In turn, there's Jack's deadpan reaction to meeting Jen after she yells at him.
Jack: She seems nice.
The dialogue between Karen Hayes and Tom Lennox in season six after he's been held captive for a while.
Karen: What happened Tom? Did you fall down the stairs?
Day 7, hour 20: Tony Almeida is double-crossed by the person he hired to collect one of the weapons used by the terrorists. The double-crosser is overpowered and dragged to the bathroom for interrogation, where he expires between scenes, with his head propped against the side of a bathtub. In the next scene, the interrogator is seen coming out of the bathroom, freshly showered — with the body still in the same position as before, evidently forgotten by the people in the room.
Day 7: Marika and her sister Rosa are having a heated argument about Marika's boyfriend Samuel. Then Jack and Renee smash their way in with guns. The look of astonishment on the girls' faces is priceless.
Watching two not-very-bright characters trying to rob a warehouse with assistance from a Voice with an Internet Connection is hilarious. They start to panic when the codes don't open the door, only to find out they were at the wrong door. "3101. This is 3110." Cue the most impatient and priceless "OMG you idiots" expression from Dana Walsh.
This scene is legendary. Jack and George Mason are, quite literally, out in the field. Jack, with Mason behind him, dramatically climbs and leaps over a fence. Mason slowly walks forward and nudges the fence slightly, allowing him to smoothly step past it. The look Mason gives Jack afterwards is absolutely priceless.
Chloe reaming out Kim's shrink Landes:
Mr. Landes: This is a dangerous situation. Tempers are high. So everybody just breathe.
Chloe: What is with you and breathing? Is that your solution for everything?
Day 7: The First Gentleman, Henry Taylor, gets drugged and can't speak properly:
"You killed my son, you killed Rrroger!"
Possibly the best part of Season 6 are all the awkward, disbelieving, if-you-weren't-my-boss-I-would-be-facepalming-so-hard facial expressions Tom Lennox makes in regard to Vice President Daniels' affair with his assistant. Then Tom is required to spend the next 40 minutes having to watch Lisa get it on with her other lover.
Tom Lennox: And... finally, we're done.
Season 7, episode 11/12: Jack is in the middle of interrogating a government official who's working for the terrorists, when the president calls and, despite his efforts to justify his actions, she orders him to stand down. Jack's responds by tasering the phone.
Nina: He is gonna put a bullet in my head before I can say hello and then he's gonna turn the gun on himself.
Jack: We'll make sure he doesn't turn the gun on himself.
Chloe offers to talk with Jack about anything he needs to. Jack reacts with a look of pure horror. Chole's face after he leaves is all, "Well, that went well."
Season 4: CTU has apprehended a suspect connected with the terrorist plot of that season. The terrorists find out about this and call "Amnesty Global" to delay the interrogation so their plot can happen. Jack Bauer's reaction when he finds out from his boss, Bill Buchanan, about all this is priceless. Especially Buchanan's response to Bauer storming away.
Buchanan: Ah, Jack! Don't make it worse!
Season 4: Mandy getting punched in the face. It's a combination of the framing of the shot (namely the entering of Curtis Manning's fist) and Mia Kirschner's facial expression.
Season 7: The FBI has tracked down a suspect whom they inform Jack is ex-SAS. Jack Bauer says with complete focus "Does he have a family?". Everyone in the room shoots him a look that's a combination of "You wouldn't" and "Oh God, he will.". Jack then calmly says that he's ex-Special Forces so he won't break unless they target his family. It's a morbid scene but the look everyone gives Jack Bauer is priceless.
Chloe: This Lynn McGill person is really slowing things down.
Spenser: Yeah, that's his style. I worked with him once before. He usually loses about 20% of his staff in the first couple of days after taking control of a new office.
Chloe: That many people quit?
Spenser: No, most of them were fired.
Tim Woods: The doctors are surprised Hodges pulled through.
Allison Taylor: That's because they never met the son of a bitch.
Directly afterwards, the secretary calls to report that Agent Walker and Jack Bauer are on the line. The president tells her to put it through before she can even finish.
In season 8, when a cocky Charles Logan spots a renegade Jack Bauer, armed with an assault rifle and wearing a body suit (complete with blast shield face mask and kevlar). Logan's 'oh shit!!!!' expression said it all, but his exasperated cries of "IT'S JACK BAUER!" made an already amusing (and completely badass) moment downright hilarious.
That was quite possibly one of the greatest moments in 24. Logan nearly in tears was just the icing on the cake!
In Season 7, there's John Quinn's gigantic Oh Crap reaction when Jack flips over the trailer he's in with the backhoe of a bulldozer.