In my opinion, the Joker is the ultimate comic book villain. From the beginning he has been one badass dude, killing with laughter in a time when comic book deaths were few and far between.
He is the ultimate nemesis, a criminal so chaotic, so insane, that Batman's logical mind simply cannot keep up. The Joker has seen his character remodeled over the years, but not as much as many of the other comic book heroes and villains that get facelifts, proving that the Joker's creators more or less had it right the first time.
ComicsAlliance has an interesting and insightful article detailing the history of the clown prince of crime, read on if you want to see just how far the Joker has come over the years.