By far Rachel's death. Considering the interrogation scene then building to Batman racing to save one of them intercut with the gorgeous piano melody with them trying and failing to cheer the other up in a time one of them is going to die is amongst my favorite moments in any film, ever, it's the easy choice. The Joker vs Batman scene sums up the entire film, then it becomes so fucking intense, tender, then devastating.
"Unlike painting and literature, the cinema both gives to life and takes from it. Literature and painting both exist as art from the very start; the cinema doesn’t."