Nemesis: Martian Manhunter
Other Major Foes: Captain Harding
Appearances: One comic story
Powers: Hearing enhancement device, electricity emitting rifle, and various other pieces of tech.
Alex Dunster was a master thief specializing in rare technological breakthroughs. He successfully fended off the Martian Manhunter for an extended period of time before his eventual capture.
Win: Wiley Dalbert (?)
Lose: Professor Arnold Hugo (8-9); Professor Anthony Ivo (4-8)
Nemesis: The Flash
Other Major Foes: Other Flash Rogues.
Appearances: 500+ comics, numerous cartoons, a live action television episode, and some video games.
Powers: Freeze gun.
Bio: Leonard Snart was a hood who recognized that he had to find a way to deal with the new super-hero known as the Flash, so he invented a freeze gun to slow the Scarlet Speedster down. Captain Cold has since established a long career as a leading member of the Flash's rogues gallery.
It has been argued that the Manhunter from Mars was the first super-hero of the Silver Age. Alex Dunster was the villain in his third adventure, his first evil scientist/super-foe, and Dunster worked on a machine tied directly to the hero's origin. It is more commonly held that the Barry Allen Flash was the first super-hero of the Silver Age. Captain Cold was the villain in his third adventure, debatably his first fully costumed super-foe, and Cold worked on a weapon tied directly to the hero's powers. If neither of these guys had appeared again, this might have been a match, but Captain Cold went on to become a fan favorite villain with a multimedia presence dating back to Challenge of the Super Friends, whereas Alex Dunster was never seen again and thoroughly trumped by Professor Arnold Hugo besides. I see Dunster attempting to swipe Leonard Snart's gear, but not surviving the experience.