The last Video Girl Ai OAV, specially Ai's good-bye to Youta, how he actually gets into the Video World to not abandon Ai, and his last stand to Rolex as he literally walks on broken glass to rescue Ai and bring her back home. The last one, with Maki Kimura's "Suki nan desu" in the background.
If you go to the manga...
Natsumi's death (specially when her Jerk Ass boyfriend Shimizu finally goes to see her at the hospital and is crushed when he arrives just moments before she dies and Moemi offering herself to Youta in despair some time after her Near-Rape Experience, but he can't accept her offer and they hug and cry together will make you bawl.
If that doesn't do it for you, there's also the second-to-last chapter where Youta frees Ai from Rolex's control by smashing the VCR playing Ai's tape, but also dooms her to disappearing for good in the process; the two of them at long last share a tender kiss as Ai disappears into sparkling motes of light.
But even after all that, if you don't cry at the Grand Finale where Ai comes back to life as a human being and finally gets together with Youta as he finishes his picture book dedicated to her, you have no soul.