Nothing is outright hilarious in this series — you need a dark sense of humor to even read it in the first place without wincing. Nevertheless...
Courtesy of the title character:
Nny: Dear Die-ary, today I stuffed some dolls full of dead rats I put in the blender. I'm wondering if, maybe, there really is something wrong with me.
There's a time where Johnny goes to Hell. A man shows up to show him around town. After driving for only half a panel he recognizes Johnny, though the latter doesn't — at first. The man starts to yell how Johnny killed him, reminding Nny what exactly he did that got himself killed in the first place, and then Johnny starts to remember. He stares at the man...and boots him out of the car while it's moving.
Nny is arguing with the Doughboys (Mr. Fuck and Psycho-Doughboy), personifications of his self-loathing and self-encouragement manifested as ghoulishly painted pastry store display dummies. At one point, while arguing with a bunny nailed to a wall (Nny's conscience), they get up and rip the bunny's head off.
Nny: I don't remember you guys ever moving around like that.
Nny's whole conversation with Squee the first time he meets him. Especially when he starts talking to Shmee.
Nny: (at Squee's teddy and apropos of nothing) FUCK YOU, MR. BEAR! You speak LIES! LIIIIES! Stuffed with pure venom, you vile lint-infested bastard! How many more like you are there?! How many more?! You can't even imagine the things I've endured! And always at the hands of shit like you! You don't know the truth!
And then he does what Johnny does, which is rip the offending individual to pieces with a knife while screaming. In front of Squee. And then he sheepishly hands the shredded teddy back to the kid.
Squee's Dad: "I haven't smiled since you were born!"
"You stink! Focusing on the mundane! Money! Fashion! Cream cheese!"
And, of course, one of the most memorable lines in the history of the series, so much so that it gets called back to not only twice here, but also at least once in I Feel Sick, and spawned a recurring character to boot. Devi (said recurring character) is on a date with a man when he suddenly feels intense gastrointestinal pain. Not wanting to go to the bathroom, lest Devi think he's lost interest, he sticks around only to find that it's worse than he thought. And as he craps himself, his mind reels, trying to find some plausible reason to get up. He's still thinking when a nearby table notices the stench, forcing him to settle on an excuse. Panicked, he notices the poo dripping down his legs and blurts out the best excuse that occurs to him:
OH MY GOD! SOMEONE PUT SHIT IN MY PANTS!
That's not the last we see of Devi's date, though. He later shows up during Nny's rampage at the cafe:
He later shows up in the background in Heaven, complete with a snarky label courtesy of Jhonen. "Freedom from shitting in his pants!"
Devi even briefly mentions him when Nny is planning on killing her at the end of their date:
Devi: Suddenly, being out with a guy who dumps in his pants seems REEEEEALLY nice.
Tonja (re-tooled as Tenna) is trying to get Devi to move past nearly being murdered on her best date ever — or at least leave her apartment. Finally, she suggests that she call Nny's house; surely if he's that bad, he'll have moved on by now. Right? Right. Except for the part where Nny picks up with a polite little "... Hello?" Followed by a gunshot and screaming.
Nny's reaction to Jimmy. Just gotten back from Hell, somewhat sane, feeling social... and this absurd copycat's on his doorstep. His incredulous expression is just priceless. And then, the one time he decides to be patient...
Nny: Who the hell are you?
Mr. Samsa. He's most likely multiple cockroaches,but Nny is under the impression that he's a single immortal cockroach.