Joker is the most insane. He is downright crazy. He would kill you just cause you made him laugh. You ask for his help when you are his ally? He goes and stabs you in the back and decides to solo the opposing heroes himself. Especially this one time where he killed Scarecrow and the two were working together and Batman was basically at their doorsteps.
I mean basically any of the other 3 would realize that having an extra person would definitely be more help and would be useful in fighting Batman.
Deadpool, may have a daredevilish and crazy fighting style, but really when he isn't all out fighting, he is pretty laid back and just likes to talk and make jokes a lot.
The Punisher isn't really insane. He's just too absorbed into his own past and wants to run away from it, especially through killing. You could say the Hulk at full out rage is crazier than the Punisher. It's why he's a comic book "hero" and why many readers of the Punisher series can see where he comes from and relate to him.
Larfleeze isn't insane. He is just greedy and wants the whole universe in the palm of his hand. Basically he is driven by his greed and taking it in any way. But, as long as every single action a character takes is explainable, the character isn't really what you would call "crazy."
Edit: As for the thing about Super Sanity -
Joker is insane because his core self is completely sane.
I guess you could say he is suffering from a really advanced form of multi-personality disorder.
So while his core self technically disqualifies him from this thing, all his other personalities are insane.
So in other words - Joker is insane as long as he is being the Joker under one of his other personalities and not his core self.