In the manga you get Near vs Dart. During his first phonecall with Light you see Near throwing a dart at a empty dartboard but failing to hit it, and then you get a panel of the floor below being littered with darts. After he hangs up and the SPK are talking to one another you see Near has moved his chair from the other side of the room to within a foot of the dartboard in an attempt to actually hit it. A few chapters later you see Near whittling his own dart, which he throws from the other side of the room and..... it bounces off the side of the dartboard and into a corner of the room, decapitating one of his toy robots in the process.
The Yotsuba members, staring at a dozen "ridiculously cute" model girls, with How-the-fuck-did-we-get-there look on their faces.
Shimura Suguru: Oi, I think we are starting to go of a tangent here...
I think the dub had the line as something like "Is it me or is this starting to get really weird?"
Not to mention that, while the Yotsuba group are trying to keep serious, the camera pans down to show them wearing fluffy pink slippers.
Aizawa, getting more and more annoyed by Misa's childishness. You can see the pain on his face, as he barely stops himself from snapping. Then he snaps.
Misa: "Hey, Light. Wanna come sleep with me tonight?"
Light: "Wh-What are you talking about...??"
Misa: "Hahaha! Just kidding! You're saving me for after we catch Kira right! You don't have to be shy about it!"
L: "Yes, Light. There's no need to be shy."
Light: "I'm not being shy!"
L: "No need to be so serious either."
In one episode, Near is explaining his deduction of Kira's sinister plan. He then moves a superhero action figure through air. "Whooooosh."
Another one involving Near, after he's been informed of Misa's and Takada's confrontation:
Near: What this proves is... *dramatic pause* ... Light Yagami is a lady-killer.
Rester: Near, please be serious.
And note that this line comes from Near of all people.
"Since you're the deputy chief of police, it would be pointless to tell you not to contact the police."
Even more hilarious in the Latin-American dub, thanks to the absolutely deadpan delivery by Soichiro's LA-VA, who also happened to be dub!Gandalf, of all people, making the scene even funnier if that's even possible.
Most of Light and Ryuk's interactions with one another are either funny, oddly sweet, or a combination of both. Two others that immediately come to mind is them teasing one another about their respective popularity with the ladies, and the scene where Ryuk, body all contorted, literally eats an apple from Light's hand.
L steals Misa's cell phone, Light attempts to call Misa while standing a few feet away from L. Hilarity Ensues.
Misa: No way! somebody just touched my butt!
L: Uh... this is an outrage! Taking advantage of the situation is unforgivable; I will find whoever is responsible for this!
Background character: I want to touch her too!
Misa's "date" with the handcuffed Light and L, devolving into a huge fight between Light and L that's half them hitting each other and half their being dragged around by the handcuffs from the force of their own blows. "Last chance for cake!"
To those who didn't watch the video, it's Sidoh snatching the Death Note out of Mello's hands, and plopping it on Syder's head. Mello's reaction to the note seemily flying out of his hands is priceless.
That scene is excellent in the manga version as well.
Made funnier when Ryuk makes her phase right through him, and says, basically "I'm shy around girls."
Speaking of Misa glomping people, Light's reaction to her random flying glomp? "This is the first time in my life that I've been provoked to hit a woman."
The "I'd like you to take out the part that says 'you're free to kill L'" scene.
Even better, he said: "Good... but unless you take out that part about "you're free to kill L"... I will die."
Light in the 'please confine me' obvious bullshitting scene "Another me could be out killing people while I sleep!"
L's inner monologue, which basically amounts to "Are you seriously expecting me to buy this crap?" makes it even better.
This is probably unintentional, but I always found it amusing when L died in episode 25, and everyone in the room started screaming from the top of their lungs, jerking, and groaning, as if Kira would have started killing them, only to stop a few seconds later, surprised to be alive.
This tidbit from episode 34 while the task force is listening in on a conversation between Light and Takada:
Matsuda: Oh boy, Light's in trouble now!
Ide: Matsuda, is there some reason you're happy about this?.
Matsuda: Ohh, he's busted! Light's in the doghouse!
Aizawa: Shut up, Matsuda!
In one of the episodes during the Yotsuba arc, L imitates one of Misa's commercials.
Matsuda was spying on Yotsuba when he was discovered and quickly pretends to be Misa's agent, hoping to get her a gig with them. They don't quite buy it and refuse to let him leave. Meanwhile, the taskforce wonders where Matsuda is, but don't want to reveal everything, in case he's in trouble. L calls Matsuda's cell, and..."Heeey, Matsuiii! Wanna go out drinkin'?!"
The taskforce told Near they were done taking orders from him and were going to look for Light. Near makes what can only be described as a O_O face before looking away and reverting to his usual expression.
L's "disguise" in the live-action movies. Leave it to L to be the one thing in Death Note that makes you laugh.
Also, virtually every conversation between L and Misa in the manga usually qualifies.
Misa: How about if I give you the cake, will you leave me and Raito alone together?
L: Even if I leave you two alone together, I can watch from the cameras, so it's the same thing.
Misa: I told you that's perverted! Will you stop with that kind of fetish!?
L: Say what you want, but I will be taking the cake.
After Light loses his memories of the Death Note, there is a scene where he sits for a while, staring off into space, wondering if he'd kill criminals if he had the power and if he could have done it without having memory of it. Then:
L: What's wrong? You've been staring at me for the last two minutes. Wait, let me guess, you must be annoyed because I'm the only one eating cake at the moment, am I right?
They're all worrying about how Eraldo Coil is trying to find L. L explains that he's really the world's three greatest detectives, using different names in case a situation like this arises. Then he randomly hands Mogi a strawberry and says in a serious tone, "I'll give you this strawberry if you keep it a secret, okay?" What really makes this one, though, is Mogi slipping the strawberry into his pocket.
Kind of minor, but in the very first episode, Light is outlining his evil plan to Ryuk and learning about the Death Note when his mom interrupts their conversation. She takes one look inside Light's room, which has been dramatically dark all throughout the scene, and cries out, "Why is it so dark in here? You're going to ruin your eyes!" Even better, Light sets the apples she gave him down onto the desk, and Ryuk curiously takes a bite when they pick their conversation back up:
Light: I have just one question for you. Why was I chosen for this? ...
Ryuk continues munching apples happily, ignoring Light.
Light: HEY! Are you even listening?
In Episode 25 in the scene where Light finds L on the roof while a heavy rainstorm is going on, the facial expression that L has when he motions a second time for Light to speak louder is hilarious.
In the manga, the entire exchange when Light is calling up Takada when she is needed after ignoring her for years and the very self-assured, arrogant expressions he gets when he knows his manipulations are working.
Hell, Light's smirking in general have spawned plenty of parodies.
Live action film. L staking about half a dozen donuts and assorted sweets on a skewer, before taking one that he'd made earlier and offering it to Sochiro:
L: Donut kebab?
The best part about this is the way that Sochiro nervously passes it on to another officer.
In the manga, when L informs Light they've apprehended Misa under suspicion of being the second Kira, rather than feign shock that his public girlfriend could be the second Kira, he just stands there with an angry look on his face. He even continues his angry stare when L obviously feigns concern for the shock he must be going through. This is a tense moment in the story, but its pretty funny how Light doesn't even bother acting at this point.