Batman finally has enough of the Joker's ranting and bursts through one of the funhouse mirrors during the climax in an absolutely gorgeous full-page splash image, complete with an Oh Crap! from the Joker. After seeing everything the Joker has done across the story, it's incredibly satisfying to see this and the beatdown that comes next.
When it's revealed that Commissioner Gordon withstood the Joker's Mind Rape.
Awesome Art - four different pieces of art from this work have become iconic for The Joker - #1. The cover image of The Joker aiming a camera at the reader, #2. The shot of Joker behind the door Barbara opens just before he shoots her, #3. Batman and The Joker laughing together in the rain (the middle panel), and especially #4. The Reveal of the Joker after coming out of the chemical bath, with the background filled with his laughter. Most artists consider themselves lucky if just one panel out of their entire body of work becomes fairly well-known, much less iconic - Brian Bolland got four in a single book.