Ra's Al Ghul

I know most people think of the Joker in terms of top villains when it comes to Batman, but I think Ra's Al Ghul is just as dangerous and so much more interesting. He takes Batman out of his comfort zone of Gotham and puts him in the rest of the world. They're evenly matched and equally intelligent. Plus, I love the term of respect he has for Batman by calling him "Detective". I always dug that.