The Mad Thinker. Guy is such a stats genius as to nearly be precognitive. His mind is so out there he doesn't even care if he's in the slammer; he's got robotic avatars to do the field work (and take the physical risks) for him.
Going after him, or drawing his interest should test even the most serious of heroes who'd find themselves slogging through a veritable jungle of his traps, tricks and automatons.
I'd love a year long event where he wages war on Dr. Doom's various organizations and apparatus for reasons that wouldn't make any sense to anyone else.