This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Ai no Kusabi
The ending. Even though this troper had already known the ending, still cried because of how it all played out. Riki deciding to come back and stay with Iason so he doesn't have to Face Death Alone and their Last Kiss was far more powerful than ever saying the words "I love you" which neither ever say.
Katze'sCry Cute after that drove it all home even more coupled by how he had rarely showed any emotion beforehand.
For some, Guy's reaction to Riki's secret in the OVA is pure Narm but rather heartbreaking not just because of his Manly Tears but Riki's reaction as well. Then how heartbroken Riki looks feeling isolated and lonely after he leaves Guy's apartment.
Riki's reaction to Guy walking out on him in the novel which was replaced by the above scene in the 1992 OVA. After Riki gives up Kirie to the authorities without a fight, Guy is so angry he walks out on a heartbroken Riki. This is all after Riki had been looking for any reason to get Guy to stay over at his place and keep him company since he was likely never going to see him again since Iason was expecting Riki back soon.