Thawne

During Tom King's run, the heroes found out that Eobard Thawne had placed Thomas on Prime Earth so he had to watch his son live out the same brutal life as a vigilante that Thomas had.

Eobard Thawne's obsession with Barry Allen has defined his life for years and drove his insanity.

Feeling he lost too much, Flash traveled back in time to capture Eobard Thawne before he killed his mother, Nora Allen.

Once the biggest fan of the Flash, Thawne has made it his life's mission to make Barry's life a living hell.

Originally a Flash fanboy who hoped to emulate the Scarlet Speedster, Thawne's adoration turned to anger when a time-displaced Barry Allen stole his thunder and ruined his heroic debut.

The scene held an ominous tone, with Iris noting that the Negative Speed Force was bound to choose a new avatar to replace Thawne and counter Barry.

Because of that, it appears we won't have any development on the Thawne front in the remaining episodes of this season.

Eobard Thawne is one of the worst villains on this list in general and probably The Flash's greatest villain.

Luckily, while inside of Nash, Thawne has lost his speedster abilities.

No longer obsessed with systematically dismantling every aspect of the Flash Family's life, Eobard Thawne has abandoned his position as one of the DCU's biggest supervillains as his faith in the age of heroes as been reawakened.

Now going by Emperor Zoom, Thawne has recreated the experiments necessary to become the Flash over an absurdly large scale.

Once Joe arrests Frida as a result, Cisco is accosted by Eobard Thawne, now in control of Nash's body but not his superhuman skills.

Thawne certainly has his ways of surviving death, so he might be able to help Barry stick around until such a time as they can figure out a way to undo Paradox's changes to the timeline.

Nora doesn’t want to risk her dad finding out about Thawne, so she decides to go into Grace’s mind by herself.

That would be Eobard Thawne, the Reverse Flash.

Thawne was the one who suggested Nora should help her father deal with the Thinker's satellite, which changed history and meant Cicada emerged years earlier than he should have done.

Thankfully, the efforts of Mick "Heatwave" Rory and a rejuvenated team of Legends restored the timeline and seemingly erased Eobard Thawne from existence once and for all.