Generally I think the Joker is the best villains and keep with me as I say this, but throughout the comics and the movies and tv shows the Joker just seems to be on this win streak with Batman and Gotham city, let us not count injustice because, well we all know what happened there. Due to superhero morals, mainly the Justice League, the joker has kept going and going with his crimes, increasing with how dangerous and sick they get. See if Batman kills him, the joker wins by getting him to kill and break his #1 rule, but if he doesn't get killed then he keeps on killing people and doing some horrible stuff so he also wins there.
He also seems 3 steps ahead, take Batman: Death of the Family, taking out each member of the bat family and making them believe he cut their faces off, long story, it's best if you just read for yourself. There are like hundreds of other reasons, and now realising this is a long reply and probably didn't have to explain it this thoroughly.
