Professor Zoom
Hailing from the 25th century, Eobard Thawne admired The Flash as an historical figure, to the point that he wanted to be just like him. Thawne eventually figured out a way to obtain super speed and traveled back in time to meet his idol, but upon learning he was destined to become Flash’s greatest enemy, Thawne snapped and embraced becoming a villain. Thus Professor Zoom, a.k.a. Reverse-Flash, was born, and he’s become even more important to the Flash mythos ever since it was added to the lore that he killed Barry Allen’s mother, Nora, when Barry was a child, and framed his father, Henry Allen for the crime. With this status quo having already been set up in Justice League, it would be easy enough for The Flash to pit Ezra Miller’s Barry against Thawne, finally bringing him face-to-face with the man who tore his family apart.