sane joker is scary because: normal/insane Joker can, regularly, come  within inches of ending the batman, while providing literally comical  amounts of style and showmanship.
Now, the Batman has shown, time  and time again, that, with time to plane, he can take down anything  and anyone The joker is the batman's arch nemesis, thus the batman  must spend time planing against the joker, commonly; and yet the joker  always remains a threat, even with all of his nonsense.
A sane  Joker would be just as evil, just with out the inefficient, if fun ;3,  shagginess: if the joker became sane he would finally take out the  batman, and likely the rest of the league as well... all in all, without  his madness making things funny... 
Joker > Lex Luther. Luther might  still be smarter, but the Joker is nothing but insane and cunning...