Mikoko's picnic with Kaiji in the third series; the scene was so popular that they based the second anime season's ED of it.
Pretty much every time Sakazaki talks about his daughter.
Otsuki's Villainous Breakdown; he waves his fists around and shouts NO COUNT over and over again. His lackey and friend Numakuwa gives him a tired look that just screams "wow, he really has lost it this time".
Muraoka suggesting UNO as the game for his gamble with Kaiji in series 3; the fact that it's the last page of a chapter combined with Kaiji's over-the-top reaction makes it look like some kind of bizarre Cliffhanger.