This is an important matter which is still today very obscure and ambiguous. I don't think Joker is the worst Batman enemy or the one he hates the most, he's the most popular and probably the most important villain in Batman's mythology, but not the villain he hates the most. I actually think Bruce likes the Joker, and his hatred against him is just superficial. He has saved him from a sure death in so many stories (now I remember the film The Dark Knight and the comic arcs Under the Hood and Cacophony) and he always shows an active interest the Joker to continue alive. Why is it? I think Bruce needs the Joker to balance himself and avoid falling in a big depression. I'll explain it: the madness of Bruce Wayne started the night a mugger killed his parents, and since that moment he has developed a very complex psychosis whose principal reason is to relieve the anxiety and the guilt for the crime. That's the reason why, in first place, he created the figure of Batman, the super-vigilante who should've avoided his parents's homicide, followed by Robin, the representation of the defenceless kid he was when the robbery took place, and the Joker, the human incarnation of the chaos that traumatizes and menaces the Bruce's psyche. He uses Robins in his battle against crimen cause he needs to protect them from violence to calm down the feeling of abandonment he has since the night of the robbery, and when one of those Robins is killed (by the Joker, of course) that isn't bad for Batman, on the contrary, that's fantastic, like gasoline in the motor of his paranoia. A Robin murdered means a new reason to continue fighting, and the Robins can always be replaced, but if the Joker was killed... who could replace him? There's not any other criminal as perfect to Batman as the Joker, any other criminal who represents so well the role of chaos in Bruce Wayne's psychosis, that is the reason why he not only doesn't kill him but saves him from death over and over again, cause without Joker he's afraid of falling in a dark hole perhaps he can't run away from... I've arrived to these conclusion after reading Under the Hood, and I hope the responsibles of the new Batman films have in count these considerations when they write the new films (although I don't have too much confidence about it...).