This is a more difficult one to answer. It has just about the same difficulty as the most epic scene ever.
First Place: Cowboy Bebop finale
We go through the entire show and find out the real reason Spike is searching the solar system: he's looking for his long-lost love, Julia. When they finally find each other, she is murdered in cold blood by Spike's arch-nemesis' cronies. There's a touching display of love in her last moments, most of it unspoken but wonderfully connoted through the visuals.
Second Place: Metropolis ending
Tomorrow's prompt: "favorite slice of life anime"