Yu Yu Hakusho/Funny

Yusuke using the body of a high school girl to kick the butts of a gang of bullies as Kuwabara was studying for the "test".
Chu...just...Chu.

"What did you say!? 'The substitutes is just a stinkin' drunk'?! Take that back! I am not a substitute!"

Hiei has his moments too.

"I know as much of games as I do of hugs and puppies, and care for them even less. Wake me for the end of the world."

- To Botan (about telling Yukina's secret): "You talk, you die."
When Puu hatches...everyone's reaction to it. THE WHOLE gang! This troper's personal favorite was Hiei's deadpan comment about the adorable creature.

Hiei: Gift from your girlfriend?
Yusuke: No! That I could get rid of!

- I personally perferred the lines from the dub. Hiei was so dry and deadpan he didn't even change his tone when he switched his attention from Kuwabara to the cute creature on Yusuke's head.

Hiei: Yes well I'm sure your minty fresh breath was well worth the wait...*looks at Puu on Yusuke's head*...What is that ridiculous appendage growing out of your head?

Yusuke: It's just a puberty thing, okay?!

Yusuke, you gotta admit, that smart mouth of his always brings something funny.
Kuwabara when he's making a fool of himself....poor dude but I still can't help but find it hilarious.
Kurama + Stealth Insult = lolwin!
- "And to think one day they could be in-laws."
When Yusuke confronts Yomi, Kurama and the Dark Tournament fighters have a silent discussion by writing out their words, since Yomi would probably hear their whispering. At one point, Rinku holds up a paper with nothing but Elipses on it. Chu Dope Slaps him saying you don't have to write a dramatic silence out. Then, when Kurama asks the other's whose side they would be on if it came down it it, nearly every one of them held up a single card reading "YUSUKE" (Rinku: "Did you even have to ask?")... Except Shishiwakamaru, who holds a paper reading "Undecided". (The Beautiful) Suzuka comes up and changes the paper to supporting Yusuke. Even more amusing in the dub, where the voice actors provide translation for the messages.
Botan and those three guys conclude that Yusuke and Kuwabara must have been an old married couple in a past life. So... "Who was the husband and who was the wife?" Among many others.
In-between rounds at the Dark Tournament, the reflection of Yusuke's inner self turns out to be... a little blue and fuzzy penguin. "Puu!"
The Genkai vs. Suzuka fight.
- This troper kept giggling at Kurama's reactions during the whole fight. For the usually unflappable Kurama to look utterly bewildered was a sight to behold.
  - The gem for this troper was Genkai's comments on the deal. Specially her comment in such a annoyed and unimpressed tone..."Fruitcake!"
All four of them talking about what they'll do when they get back to normal life after the Black Tournament, Yusuke "Skipping detention." Kuwabara, "Video games!" Kurama, "My human mother." Hiei, "Various crimes." Hiei's deadpan really kills it.
Yusuke storms out of his house, because Sensui is endangering the world and his posse is MIA (Kurama), screwing it and getting out of there (Hiei) and at a Megallica concert, yelling that he's going to the arcade to play video games. Keiko is all concerned, but Genkai shakes her head and says not to worry, because she's sure Yusuke has sensed Kuwabara's about to awaken and manifest a new power, and has gone to look after him. Cut to Yusuke angrily beating a video game. Later, we get a scene of Genkai throwing him out on his ass, screaming at him for not going to protect Kuwabara.

Yusuke: [paraphrased from memory] "I SAID I was going to play video games! What part of that didn't you get?!"

Any time Yusuke, Kuwabara, and Hiei start arguing, with Kurama mediating, or anytime Kurama and Hiei sit back and make comments while the others are doing something, mostly because Kurama's one liners are hilarious, and he's the only person who can make fun of Hiei with impunity.

(Hiei gives Kurama the stone his mother gave to Yukina, with the intention that Kurama gives to Yukina instead of Hiei) "I'm sorry, I can't accept this Hiei. You know I respect our long friendship... but... I just don't like you in that way." (Hiei's reaction is a straight up Crowning Moment)

"Is this your girlfriend, Kazu-san?" The manga is funny because Kurama gets all pissy. The anime is funny because he's so nonchalant, even when one of the girls bends over to stare at this chest. Just to make sure.
The scene where Kuwabara revealed he had faked his death at the end of the Dark Tournament saga left This Troper in stitches, especially since it happened immediately after an epic Wangst moment by Yusuke.

Kuwabara: Reporting for duty, Captain Faker! That's me!

- And what Yusuke's pummeling towards Kuwabara afterwords. Hiei actually looking confused at this should seal that as well.

Kurama: Yusuke, if you keep it up you'll actually kill him.

After Yusuke comes back to life thanks to his demonic genes, he claims to be an evil and powerful demon in front of the Spirit World's Special Defense Forces who were sent to destroy his body and prevent his revival. The characters are quite convinced until he reveals that he's kidding.
Hiei's Comically Seriousness makes for a few good ones:

Hiei: If I wake up and find we've lost, I'll kill you all. * collapses*
Yusuke: That's Hiei for you. He faints, but he still makes time for threats.

How about Yusuke's "diplomatic" approach to Yomi in the Three Kings saga?

Yusuke: YOMI!! YOU SON OF A BITCH!! YOU HEAR ME?! I'M COMING! SO BOIL UP A KETTLE OF TEA!!

Practically any time Jin opens his mouth past his initial appearance, especially in season four. In fact, the anime seems to use Jin to make a point that scenes during that season are getting serious when they show him actually not talking for a change.
The Dark Tournament fighters after Yomi asks Kurama how he helped them become so powerful and Kurama downplays Genkai's training as eating right and working out, so to speak. The fact that it's Touya that actually starts the mouthing off just makes it even better.
When Sensui calls for Makihara:

Mitarai: That's Gourmet.
Yusuke: Oh great, does he cook?

When Kurama, Kuwabara, and Botan are looking for Hiei, Botan whips out a "demon whistle" which will make demons come to it like a dog whistle. The range was practically the whole city. When Botan blows it, it turns out Hiei was sleeping in the tree forty feet away.
For shame tropers, why has THIS not been up here yet?! When finding out the demon Miyuki is actually a transvestite...Yusuke's deadpan expression matched with this quote.

Yusuke: The family jewels have not been stolen.

Or this moment...

Thug 1: (to Kuwabara) You're pathetic.
Thug 2: You may as well give us your backbone too.
Yusuke: (thinking) Whoa! This Eikichi must be something special for Kuwabara to sink that low!
Thug 1: (pulls a cat out of a bag) I just can't believe you're doing all of this for a retarded fuzzball.
Eikichi: Meow!
Kuwabara: D8 AHHH! EIKICHI!
Yusuke: Dwuah?! (face faults)

A face off for the ages! Kurama vs Kuwabara vs Hiei...in Rock, Paper, Scissors!
Yusuke vs Jin had quite a few of these moments...mostly from Jin.
- "I thought I was crazy! But you take the prize Urameshi! The hell was that?! You don't just make bombs go boom in your face!!"
Episode 4 after Keiko got Yusuke's body out of the burning house. When Kuwabara looks inside the blanket to see Yusuke's corpse, the look on his face was priceless.
Yukina's last line about the Ship Teasing of Yusuke and Keiko during an intermission for the Dark Tournament. Having barely any experience with the outside world, her confusion about the subject is understandable but hilarious.
Genkai when she greets Yusuke after the Dark Tournament.
When they are all on the boat, heading to Hanging Neck Island for the Dark Tournament, They announce there will be a Preliminary match on the ship. The "Masked Fighter" volunteers to fight all the other demons on the ship. Kuwabara insists against it, saying they don't know jack squat about the guy. Saying that "he might lose" then what would they do? This following exchange happens.

Hiei: *smirk* If that happens then we'll just have to kill everyone on board. We'll say we're the right team and no one will complain.

Kuwabara: *creeped out expression then says in a complete deadpan* You didn't have many friends growing up, did you?

Kuwabara insisting that he's not going along for the next adventure no matter how much Yusuke tries to make him (Despite the fact that he volunteered to come along during the St. Beast Case, as Yusuke pointed out)...cut to Kuwabara seeing Yukina's face, the guy goes completely crazy...and insists on going to save her.
- It gets even better, Kuwabara insists that they are attached by the "Red String of Fate" and will lead them to Yukina, not to mention his "headband of love!"
  - Or his reaction when one of the demons in the Triad, cuts his "Headband of Love" off.
- Yusuke telling Botan that (thanks to Kuwabara seeing Yukina and instantly switched his affections to her) Kuwabara seems to have moved on and forgotten about her...Botan says that she's broken hearted for being replaced.
When Hiei greets Yusuke before the preliminaries of the Dark Tournament both moments were this...though the edited toonami version of the dub, actually managed to be funnier than the uncut version!

(After Hiei attacks Yusuke with his sword, Yusuke dodges the slashes at rapid speed, Yusuke manages to finally catch the blade...)
Uncut Dub version-Yusuke: Damn Hiei, this how you say hello now?
Edited Dub version-Yusuke: Hiei, I see you've been working on your people skills.