And explain why, cause I personally believe it's Ra's Al Ghul because of his resources, his immortality, and his heavy influence on Batman's history. Like in the movie Batman Begins as his mentor training him to become The Dark Knight. And in the comics, Talia Al Ghul is the mother of his son Damian Wayne. That's a deep problem when it becomes a matter of family and knowing ur greatest weaknesses. But I'd like to believe Two-Face is a close second also due to being Batman's former friend and Harvey having a burning hatred and rivalry with the Robin's in the comics too.