Professor Zoom

Archenemy of The Flash and Kid Flash, Eobard Thawne became Professor Zoom (or Reverse-Flash) and due to his super-speed, he is very hard for the Flashes to defeat.

Origins
In the 25th Century, Eobard Thawne was a fan of the Flash (who was dead at this point) and gained super-speed like The Flash had. Then, Thawne travelled back in time using a "cosmic treadmill", which broke down and sent Thawne several years after Barry Allen's death. Visiting the Flash Museum in Central City, Thawne saw the statues of him dead in The Flash's hands. Thawne then realized he was destined to become Professor Zoom, and was destined to be defeated by his own idol. After his first fight with The Flash, Zoom was knocked forward in time, and returned to modern day again (angry at The Flash) to finally destroy The Flash and Kid Flash.