In the late 1940s, batman was introduced to one of his most crazy but bland enemies, MAD HATTER.
Wait? The crazy guy from Alice in Wonderland? Technically, Batman's Madhatter is Jervis Tetch, a scientist obsessed with Lewis Carroll, the creator of Alice in Wonderland.
Some versions of Mad Hatter has him fall inlove with a woman… named Alice… yeah…
So what makes him a threat? Besides a large hat, Mad hatter has mind control devices… but that's pretty much it…
Despite being over 70 years old, only the biggest batman fans know who he is, but he has made many appearances in tv shows. He's also in Lego Batman.
He has genius level IQ, but has 4 major mental conditions. Major immaturity, Delusional disorder, Obsessive–compulsive disorder and Schizophrenia.
However, Mad hatter is hated by fans due to not being a big threat and not being an original idea.
http://comics.ign.com/articles/622/622304p1.html