Harley Quinn's Perfect Avenger Combines Every Gotham Villain (Literally)
Harley Quinn's greatest foe appears to be a villain who combines every member of Batman's rogue gallery into one dangerous and creepy form.

It looks like the worst enemy Harley Quinn faces is a terrifying combination of Gotham's worst villains. Dark Nights: Death Metal Trinity Crisis #1 sees Harley face off against the perfect foe who embodies one of her worst fears.
While Harley's career in the DC Universe began as Girl Friday, the Clown Prince of Crime, she has since grown into a character of her own, far removed from The Joker. Since setting out on her own path, Harley has traveled all over the world, even to the distant New God Apocalypse. But while she's still finding her own balance between hero, antihero, and villain, she's had some trouble getting her own roguelike gallery. She's faced many adversaries, sure, but few villains can be successfully labeled as one of "her" arch-enemies.
That said, Harley meets a villain who is her perfect nemesis. In Scott Snyder and Frances Manapoor's Underworld: Crisis of the Death Metal Trinity Issue 1, Harley teams up with Batman, Superman, Wonder Woman and Jonah Hex to infiltrate Batcastle to stop the Perpetua. Unfortunately, Laughing Batman's army of dark knights is the biggest problem. Even after passing the initial wave, the team was discovered and attacked by a group of Dark Multiverse variants. Harley in particular has to face the Ark, a creepy combination of Arkham Asylum's worst villains. He wears a scarecrow suit, uses Mr. Freeze's techniques, and his face resembles Killer Croc's. Ark also has arms like Clayface, and uses them to attack Harley by forming sharp clown poker cards. The villain insists that Harley be a part of him, insisting that he is where she "belongs."
Ark Represents Harley Quinn's Number One Fear

Like most inhabitants of the Dark Multiverse, the Ark has a deep-seated fear of Harley. Mainly she's not good enough to rise above her status and she'll always be a Gotham rogue. While Harley has made some major leaps and breakthroughs since her days as a tough villain, it's clear she's skeptical of her ability to move past her past. Having to confront these doubts in a physical performance made Ark an inspirational enemy for Harley Quinn.
All of Harley Quinn's regrets about her past began at Arkham Asylum. its The place where she first met the Joker is also where she spent quite some time as a villain. In addition to being an interesting rogue who combines Batman's best foes, fighting villains like Ark allows Harley to work out her problems and help her move on from her evil past. The Ark may be a resident of the Dark Multiverse, but a character return and future confrontation could be an interesting direction for Harley Quinn.