A surprising highlight for DC recently has been their Injustice comic.
Tying into the video game, the series examines the beginning of Superman's decent into villainy. His start of darkness occurred when Joker used hallucinogenic gas to trick the Man of Steel into killing Lois, his unborn child and most of Metropolis. Finally broken, Superman responded by punching right through Joker's chest.
Obviously, Joker's death hit Harley pretty hard and a lot of the series focused on her trying to move on. It also highlighted just how messed up their relationship was.
In one heartbreaking scene, Harley confessed to a pregnant Black Canary that she had a daughter. Thankfully, even Harley knew what kind of a father Joker would be and so, as soon as she found out she was pregnant, she left him for a year to have the baby in secret. When she came back, Joker hadn't even noticed she'd left.
It's far from the worst thing on this list, but it's the kind of emotional neglect that defines the day-to-day of Joker and Harley's relationship. The Joker only has eyes for one person and that's Batman.