The best replacement for harley quinn is catwoman.

The best alternatives to harley quinn are rated as following:
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★★★★  poison ivy
★★★★  batgirl
★★★  joker
★★★  mercy graves
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★  mystique
★  joker injustice
★  batman arkham asylum
★  catwoman injustice
  nightwing injustice
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Speaking about harley quinn, one probably shouldn't forget about joker.

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