I also love how in B89, we got to actually see the moment that The Joker became The Joker.
I don't mean him falling into the vat of chemicals, because we've seen that depicted before in various ways. Rather, the moment after his "surgery" when he takes a look at himself in the mirror and just snaps. If the surgeon had been able to repair his face in a way that looked relatively normal, or if his skin hadn't been white and his hair green, Napier might have taken a look at himself and then walked out of there as the same man he once was. But instead, when looking upon his new appearance, it permanently fractures his psyche and pushes him over the edge of insanity.
[YT]watch?v=QiAonyJIV_A[/YT]