Depends on which Harley! For Harley Altmeyer, it would have to be one of his sisters, though I'm torn on whether Misty or Amber would be better (for me, worse for him). For Harley Quinn, it'd definitely be Pamela Ysley/Poison Ivy, because she misses her so much, although I'd be delighted to see Kate Kane turn up (HINT HINT, SCOTT).