Harley Quinn actually escapes from the Joker and lets herself be put into prison. This is in light of the fact he was going to kill her off like he did to so many other Harley Quinns before her (assuming the Joker wasn't lying).
The Joker has one with his return. He first cuts the power to the GCPD and then proceeds to murder 19 police officers all while making bad jokes and getting his face back before Batman gets the power back on. Terrifying to the extreme but also damn impressive considering he's just one man with no superpowers or gadgets.
Harley attacking the Joker in general. Her moving past her obsession with him and only doing what he said for Deadshot is pretty awesome. The fact that she had more success than anyone else aside from Batman helps.