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...