I vote for the Joker because I enjoy watching the Joker as a villain. I am embarrassed of Vader when I see him now.
Vader would brute force his way through things, brooding about it afterward, wheezing because he's out of shape. Vader is a bull-headed geek. He probably hasn't showered in 30 years. Maybe if we weren't counting the prequels, maybe I could have him in the running. But Vader is nothing more than an intimidating presence. He never does anything!
The Joker would trick people into death traps, laugh at them, torture their families, then make his escape, letting the hero get away so he could do it all again, assuming the hero's mind hasn't broken and landed him in Arkham Asylum. The Joker is an evil genius. He's scary. He is the modern classic of insanity, of the darkest and cruelest urges of humanity.
Vader is a whining prat.
And, let us not forget, Mark Hamill, the greatest actor to portray the Joker, already beat Vader once.