The explosion of roses in their battle. That was just beautifully well done.
Angelic Layer's 25th episode is a tearjerker the entire way through when Misaki realizes her mom, who she hasn't seen in years, is Athena's deuce. She then spirals into a depressed state thinking that her mom didn't want to see her and she regrets saying that it's alright all these years. Even more saddening is how her mother Shuuko tries to find Misaki but then she starts crying when she realizes she doesn't know her own daughter. Finally when they meet, Misaki begins to cry in the middle of a crowded walkway. The episode ends on a happy note and then makes you cry for joy. The show has a few other sad moments, but that episode takes the cake.
Episode 16, which featured an absolutely straight use of Ill Girl in the form of Sai Jounouchi's younger sister Rin. You'll never feel sadder to see one of Misaki's opponents lose, particularly the final flashback where we hear Rin, in whose memory Sai has been fighting for, say "I'll always be with you, Big Sister!" To top it all off, we learn at the end of the episode that the secret to withstanding adversity is "To cry as much as you want to, when you need to cry."