The Joker is no joke, but Luthor is Luthor. He is basically the Joker with less lunacy, a meaner streak, and a penchant for expensive suits. And while the Joker is near-genius in intellect, Lex is genius.
And were it not for Bruce Wayne, Lex Luthor would probably be the most dangerous man on the planet (although Val Savage and Rhas Al Ghul would probably object).
No offense to Harlequin's boo, but the guy who makes Mercy cry mercy is the man.