When Data 7 was finally able to remember both his own past, and his friend Cyber 6.
While his fall was a tearjerker at best, Cyber-29/Data 7's memories of his sister were aww moments. One special moment, was prior to his fall, he wanted to get a pretty flower for 6 that was on the edge of a cliff.
In the final episode, Cyber Six is about to go off to fight the enemy, fully expecting to die in the battle. Before leaving, she stops by Lucas's place to thank him for everything. He sees how anxious she is about something and moves closer to her, at which point she finally confesses to not being human (something which she's spent the entire series worrying about). He doesn't care at all, and they finally kiss!