6. Poison Ivy
For no other reason than her role as a femme-fatale. The argument could be made that both Catwoman and Harley Quinn fill this role, but I dont think that either really pose as grave of a threat to the Caped Crusader. Theres always been that sense of sexual tension between Selina Kyle and Batman, and depending on which storyline (Hush for example) depends on whether or not she knows that he is Bruce Wayne. Harley Quinn has always been easily defeated due to her unrelenting devotion to the Joker, something which the Dark Knight can easily manipulate into his favor. So bring in Poison Ivy to flesh out the female section of the characters. Think of the match ups that you could pit her against Wonder Woman, Harley, Hawkgirl. Or even who she could form an alliance with Swamp Thing anyone? What about Bane? (Sorry, couldnt resist). The way that she would play would make for some awesome matches against Wonder Woman assuming Ivy could use vines as some sort of whip against the Lasso of Truth