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.
Anytime 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
Kurama: 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
Koenma:(as a casual introduction) "Hello. I'm Koenma. I'm very cool."
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."
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.
In the dub, Yusuke's reaction to finding out the guy giving him his ordeal looks like a little kid. The response to said teasing is also hilarious.
Actually, I'm proud to say that in addition to knowing the greatest secrets of the universe, I'm also quite potty trained.
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.
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".
Yusuke trying to get Keiko to recognize him in Kuwabara's body and finally settles on a...unique version of "Guess who?". It gets even better afterwards, where Keiko is hanging on Kuwabara's body as Yusuke's spirit leaves it... Kuwabara's bafflement just makes it.
Even funnier is when Keiko figures out that Yusuke's not possessing that guy's body anymore, and he's still hugging her. She freaks out and slaps him, before quickly apologizing and running away. Kuwabara just looks completely confused and mutters "No, it's fine. I probably deserved it".
When Yusuke finds out about Eikichi:
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."
The manga has a chibi 3-part doodle showcasing how Yusuke discovered that Miyuki is a transvestite. The first part shows Yusuke grabbing Miyuki chest and discovering it was padding. The second part shows him grabbing Miyuki between the legs and discovering that "she" was packing quite a piece. The third bit? Shows Yusuke PUKING as he runs away from "her".
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 brokenhearted 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."
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 in a battle royal. 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?"
Also, a funny moment when Chu first shows up—he's staggering around the ring and acting like a clown, but Shizuru points out that there's something beneath his clownishness that seems highly dangerous. Keiko says that everyone must be worried and at the edge of their seats... cut to Koenma blissfully stuffing his face.
Yusuke vs. Jin had quite a few of these moments... mostly from Jin.
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!!"
Made even funnier by Jin's tendency to speak faster than Ben Croshaw. Don't let the punctuation fool you, there are no breaks between those sentences.
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.
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!"
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 preferred 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: "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."
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!"
This is swiftly followed by Yusuke proceeding to beat the everloving crap out of Kuwabara. Even Hiei is disturbed by the beating Yusuke gives Kuwabara!
Koenma: "Yusuke, teach him anymore and you'll actually kill him. That's kinda against the point, isn't it?"
Koto and Juri tend to have several silly moments over the course of the Dark Tournament Arc.
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 (Kuwabara), 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: "I SAID I was going out to the arcade Ms. Senile!"
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.
The clincher was the way this was revealed: as a Funny Background Event with Hiei falling head first from the branch he was sleeping in.
When Sensui calls for Makihara:
Mitarai: "That's Gourmet."
Yusuke: "Oh great, does he cook?"
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.
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 others 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) Suzuki comes up and changes the paper to supporting Yusuke. Even more amusing in the dub, where the voice actors provide translation for the messages.
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!!"
The Dark Tournament fighters after Yomi ask 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.
After the preliminaries of the last tournament end, a Big Bad Wannabe goes up to Yusuke and tells him that he's going to defeat him in the second round, Yomi in the third round, win the tournament and take over the Demon World. Is this where Yusuke fights and easily defeats him in the second round? Nope! The demon doesn't even make it past the opponent in the first round, who in turn goes on to lose to Yusuke (presumably fairly easily), and Yusuke loses to Yomi, albeit barely, showing just how much of a blowhard his would-be second round opponent is.
"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.