Xenovia: (Switches straight back to her normal-unfeminine voice) I know, I tried to copy Irina, and it was SO difficult.
Xenovia trying to get Issei to sleep with her and is completely stone faced about it.
And she continues these antics next episode, first by holding up several condoms in class while asking Issei for some "practice".
Serafall's antics during Class Observation Day (and subsequently, Sona just losing it out of sheer embarrassment).
Koneko trying to toughen up Gasper by offering him some Garlic.... he's a vampire mind you. The sight of her chasing him◊ all the while chanting "Here Here Here" in her usual deadpan tone is a side-splitting sight.
During the conference, Azazel asks for Issei's opinion (who says he can't make that serious an opinion so quickly). So he puts it in Layman's Terms: if there's peace, he can have sex with Rias every day; if there's war, he can't... all while Rias is present beside Issei.
And needless to say, Rias's reactions to this are pretty funny as well.
He also uses a similar analogy in order to make Issei power up in his fight against Vali (Half Dimension reduces everything affected by it in half, including Rias' breast size). It works so well he even lampshades it.
What the heck? Seriously? His powers skyrocketed just by the thought of his mistress' tits shrinking.
In the same episode, when Gasper has frozen his captors in time with his newfound powers, Issei wasted no time at all Dress-Breaking them, and was about to pounce on and grope the helpless villains... when Rias stopped him with a Megaton Punch to the head.
Any time Koneko performs some feat of Super Strength accompanied with a tiny little "Eh."
Light Novel (Main Story)
In chapter 2 Issei suffers a Rhetorical Question Blunder when he fails to use the magical diagram after being summoned and Rias tells him to travel on foot. He asks "What kind of Devil shows up on a bicycle after being summoned?". Koneko points at him in response.
Chapter 16.5: "I HAVE ABSOLUTELY NO DERE!"
The reactions of Issei's comrades to Dress Break. Koneko tells him to stay far away from her, and Kiba actually apologizes to the other team.
Kiba gets spanked by Rias with Issei laughing throughout the whole time.
Though that might be because Ise got spanked earlier....
During the fight with Kokabiel, Ise says that he won't let him destroy the town before he becomes a harem king. Kokabiel laughs and offers to let him sleep with a bunch of the fallen angel girls. Issei is so shocked Rias has to snap him out of it with a slightly jealous reprimand.
Issei is losing the fight with Vali by a large margin. In an attempt to rile Issei up, Azazel mentions that Vali can halve peoples breasts. Three seconds later Vali is spitting up blood and struggling to stay in the air.
The Sitri teams reactions to Billingual
Volume 6, Chapter 2: Vali and Bikou with Issei look at Mil-tan. Their reaction? They think he's a nekomata using senjuutsu to hide his form. Issei's first-person tsukkomis are priceless.
Bikou trying hard not to laugh too much by the events of getting Issei out of Juggernaut Drive. And the reason why Rias' Embarrassing Nickname "Switch Princess" came to be.
Vali's Rhetorical Question Blunder where he asks Bikou if Rias' breasts are a control switch for Issei in which Bikou bursts out laughing so hard.
Volume 7, Ddraig and Albion crying at the fact that Ddraig is now referred to as the Breast Dragon and Albion being the Butt Dragon.
Volume 7 has Issei trying to talk to Akeno's breasts instead accidentally summons the freaking god of breasts! He then asks Taninim as to whether a breast god exists. EVERYONE stops fighting and looks at Ise like he's going crazy. The kicker as well was Ddraig backed him up and everyone thinks Ddraig lost it.
Volume 9: The reason why there are a lot of molesters in the streets? It's because of Issei opening a box way back in the beginning of the Volume.
Three words: Breast Lover Zombies.
Rossweisse vomiting while fighting off against Heracles because she's still affected by the alcohol she drank earlier in the day.
Ophis asks Ddraig if he wants to be the Breast Dragon Emperor instead of the Heavenly Dragon. Cue Ddraig going into another one of his Heroic BSODs.
Ddraig needs medicine to calm himself down. One of the most powerful dragons needing medicine just to calm down whenever someone talks about breasts.
During the promotion test, the practical portion is revealed to be a duel between two applicants. Issei, being Issei, is worried about how well he'll do, and treats it like a life-or-death battle. Five seconds later, he lands one punch and sends the other guy through the wall. Both Akeno and Kiba were cracking up when they heard about it.
Volume 12: Opfis and Great Red sang the Oppai Dragon theme music. Ddraig was definitely not amused.
Issei, in his spirit form, laments at the fact that he can't make out with Rias with his current state.
Issei's He's Back moment. He even has to say "breasts" to let his friends know that he has returned, much to his consternation.
Ophis tapping the "ground" that she and Issei are on. When asked, all she said was "I'm defeating Great Red."
And everyone's (From the past users of the Boosted Gear to Great Red itself) last words to Issei "Zoom, Zoom, Iyaaan~"
Volume 14: Poor Issei, he can't even sleep on his own bed anymore thanks to the girls sleeping on it. Xenovia kicks him out while asleep.
Ophis sleeping at Issei's bed like a dead person in a coffin.
Also from 14: Issei's lap has escalated into a battleground with Koneko and Ravel vying for complete control and ending with Koneko and Ravel on Issei's left and right thighs, respectively. It began with Koneko acting like it is her territory while Issei muses to himself that it must be the cat part of Koneko that makes her want to keep Issei's lap as hers.
Xenovia is having a hard time mastering all of the Excaliburs due to being a 'power-idiot', as Issei puts it so Irina tells her that she might become the self-proclaimed swordsman. Xenovia doesn't take well to this and retaliates by calling Irina the self-proclaimed angel. Irina is offended and asks Issei to assure her that she is an angel, reasoning that since he's her childhood friend he'll agree. Issei comments that it's easy for him to forget that they really are childhood friends to which Xenovia takes advantage of by calling Irina the self-proclaimed childhood friend, to her chagrin.
Even if he's currently the enemy, Euclid making a lot of commentary with such seriousness is just hilarious.
Volume 15: Ophis, while sitting on top of Fafnir's head with Rassei sitting on top of her head, saying, "I, with this three-combination, think, can challenge Great-Red."
Ddraig refusing to acknowledge Fafnir that Issei nicknamed the Pantsu Dragon King.
Xenovia and Irina competing over the right to sit on Issei's lap. Also, Ravel's disappointment of her status as a latecomer and Sister Griselda arrival adds to the humour.
Azazel's thoughts that Issei is better off than Vali when comparing the fact that Issei gets three meals a day prepared by his loved ones while Vali's entire team eats nothing but instant food when Le Fay is not around to cook for them. Also, the fact that a certain god-killing wolf is being given cup noodles to eat.
Azazel telling Bikou that he's considering adding Vali to the Oppai Dragon show.
Anytime Irina has her wings flickering from angel to fallen angel wings.
In the same short story, Ophis just ignores the entire epiphany about Issei and Grayfia being together and just says that she wants to swim. This was not lost on Issei.
The girls from the Occult Research Club and the Student Council being forced to audition themselves as magical girls alongside with Serafall Leviathan, with all the girls wearing magical girl clothes. Also, Gasper and Mil-Tan actually joining the audition and passing the preliminary auditions.
The Church Trio actually buying an eroge for Issei.
YMMV on this one but one of the latest short stories involve Akeno having a S&M with her father after finding out that he and his wife engaged in that sort of thing every single night. Oh and Azazel joins them, being also a masochist. In short, All Women Are Doms All Men Are Subs.
Rias asks a question as to what Ophis wants for her room since she's permanently staying at Issei's place. Her answer in a deadpan expression: "Great Red's neck."
Azazel being a salesman employee after resigning from being the Governor-General of the Grigori fallen angel faction.
Rossweisse fighting against old women just for clothes on sale.
Ophis' going through the Lost Child Notice and pretty much describing how the Occult Research Club and some other people who they look like broadcasted throughout the entire department store: Crimson-haired big boobed mom (Rias), lecherous faced dad (Issei), normal-sized boobed golden haired sister (Asia), idiot sister with brute strength (Xenovia), self-proclaiming angel sister (Irina), two birds and a cat (Riser, Ravel, and Koneko), one more mom with big breasts (Akeno), sister looking like she has low happiness and is poor who is silver-haired (Rossweisse), and a handsome guy, cross-dressing guy, and glasses girl (Kiba, Gasper, and Shinra). The sheer embarrassment that ensued pretty much took the cake.