While The Joker may be considered Batman's arch-enemy, another strong contender for that title is Ra's al Ghul.
While he may stick to a code of nobility and is even the grandfather to the current Robin, Ra's is also the head of the League of Assassins and has proved himself to be a deadly opponent in his attempts to wipe out humanity for what he sees as the greater good.
His fights with Batman have been some of the Dark Knight's most trying and even when Bruce does manage to come out on top, the rejuvenating magic of the Lazarus Pit means that the near-immortal Ra's will always be back for more.
Of course, the downside of being nearly immortal is that Ra's has had plenty of opportunities for embarrassment over his long lifespan. With dubiously effective schemes and messed up family relations, he's definitely dropped the ball more than a few times.
DC would love it if some of these missteps were swept under the carpet and forgotten about, but that kind of compliance doesn't make for a very entertaining article so let's roll up that metaphorical rug and take a peek at the dirty secrets of The Demon's Head.