As a kid, i grew up watching "batman the animated series" and one of my favorite supervillain role models became the Joker. I love his maniacal laughter, twisted outlook on life, dark humor, and sick personality.
I was disappointed whenever he failed at accomplishing his mission. The crazed persona under the clown gimmick was very realistic and original. I enjoyed reading his origin in comics like "the killing joke." It's traumatizing for a once married man crippled to the point of becoming the Red Hood and dabbling in crime only to run in fear from the Batman. Eventually, it seems that it is the Batman's mistake in creating the Joker for the criminal he is.
The Joker has done many things that make you question who the real villain is. Ultimately, what i find most cunning about the Joker is that he knows the Batman will not kill him out of justice. The Joker takes advantage of that and does all the evil he wants towards Gotham's citizens.
As far as i know, no supervillain can match the Joker's cruel and sick nature. For that, he will continue to thrive in the DC universe.