Shermie vs. Bryan's special intro in KOF'98. He keeps shouting "WOO! WOO!". This annoys her so she walks over and SLAPS THE TASTE OUT OF HIS MOUTH.
Billy's special opening against Iori is understandable, given what happened at the end of '95. What's less expected is Eiji Kisaragi teleporting in to shake his fist at Iori and then teleporting away before he's spotted.
Having Team Kim fight against Team Yagami in KOF XIII Story Mode, in which Kim tried inviting Iori to join his Tae Kwon Do dojo, and even prepared a taekwondo gi just for him. And if not enough, he also invited Hwa Jai and Raiden to the sport as well (He offers the Chang-sized gi for Raiden). Hilarity at its finest.
Vanessa: This is Vanessa. I'm investigating the area around the stadium...but there's too much ground to cover. I haven't found anything. I need something specific to go on. I can't just search an area this big for anything "out of the ordinary." I feel like a lost tourist.
Mary: This is Mary. I'm in the slums, dealing with an...unsavory individual. (Screenshot reveals this to be Duck King) I haven't come across anything of note.
XIII Team Fatal Fury Arcade End: During the victory celebration, both the Bogard Brothers are congratulated by Mai (who embraces Andy) followed by Mary with Anton (who approach Terry). As Joe takes note of both of them being fawned over by girls, much to his jealousy, he is seemingly embraced by a woman from out of nowhere. He cheers initially until he asks her who 'she' is only to discover it to be the same transgender man he saved in 2003 who is unfortunately looks nothing like a woman with the stubble, despite the makeup. Cue horrified Joe trying to run off, got caught and end up looking like he's Deader than Dead...
The Women's Team Ending has several of the Action Girls in the series partying in the Pao Pao Cafe, almost all of them drunk their butts off. (Save for Hotaru and Hinako, who are too young for drinking so they eat cake instead.) It ends with the drunk ladies (Mai, Yuri, Jenet, Malin, Xiangfei, Kasumi and King herself) openly squabbling like highschool girls over themes like their strengths in battle and their own love lives. In front of the totally embarrassed Hinako and Hotaru and a weeping Richard Meyer
Chin and/or Kim getting Kyo all pissed off via asking him if he actually did finish higschool or not;
It's even funnier if you go to Story Mode, play as Team Japan and then find the Psycho Soldiers. Athena naively asks Kyo if he did graduate or not when he asks about her uniform, Kyo is at a loss of words despite Benimaru telling him it's fine, Kensou is all "WTF?!"...
While the first is removed in XIII Climax, the latter is even funnier. In the original, when Athena asks why would he keep such data, Maxima calmly says he's just interested in her Action Girl strength. In the new version, it becomes a Suspiciously Specific Denial in which Maxima totally didn't store pictures of her for non-fighting purposes. (Hint: yes, hedid.)
King asking Andy to pay for Terry and Ryo's tab at her bar, which makes Andy really damn angry.
The third volume gives us Kyo and Benimaru meeting up with the Women's Team after defeating them. Then, Yuri and Mai start teasing Kyo about Yuki since they now know that the Japan Team fought them to save her. The usually unflappable and cocky Kyo goes beet red and then furiously stares at a retreating Benimaru...
In Ryo Takamisaki's KOF: G, the rendition of Joe's infamous "mooning" taunt while fighting Iori, Mature and Benimaru.