It's no competition really; the Prince Clown of Crime is beyond any conventional understanding of the word 'insanity'. He's had many origin stories, ranging from being a failing comedian to a victim of an abusive home, yet even the near omnipotence of cosmic forces have been unable to understand who, or what, The Joker is.
A mainstay of DC and utterly ingrained in the zeitgeist, The Joker is considered to be one of the, if not the, greatest villains of all time. Totally messed up and insanely ingenuous, The Joker is the kind of character who's unpredictability means that no one can see his next move coming, though it's sure to be one fraught with death and destruction.
The Man Who Laughs essentially personifies chaos and despite having no superpowers he's a formidable foe for pretty much anyone in the DC universe. One of his craziest runs was in 'Death of The Family' where he wore his own cut off face as a mask and tried to marry Batgirl, assuring her he'd cut off all her limbs with a chainsaw after the vows had been exchanged.
Sadly history has shown that counselling won't do much, as the last time someone tried that we ended up with Harley Quinn.
There's plenty of other examples of absolute insanity in comics, so let us know your favourites in the comment section below!