It is commonly said that a good villain often mirrors the hero they go up against. Whether it be their skillsets, their beliefs or backgrounds, a well-rounded antagonist must be able to challenge the hero on various levels.
Fortunately, The Flash has had such opposing forces throughout its near eight year tenure. Say what you will about the show's decline in quality, but at its best it has produced some of the greatest villains in genre TV.
The Scarlet Speedster has been put through the wringer, whether it be physically, emotionally or mentally. The various metahuman criminals that sprung up following the fateful particle accelerator explosion had different motivations and backstories that added layers to the series.
These multiple layers ensured that a fair number of these villains did not feel undercooked or the over-the-top to the point of being unrelatable, but were instead fleshed out individuals whose paths to crime and villainy were well defined and understood.
All this said, which baddie reigned supreme in the season they appeared in? In some cases the answer is brazenly obvious, but in others the answer is not as straightforward based on fan expectations, the villain's persona and other related factors.
8. Season 1 - The Reverse Flash
The series' first season is regarded by many as its best and this is due to its blend of light-hearted thrills and more intense/sombre fare. The latter was best embodied by Eobard Thawne/The Reverse Flash.
The series' most well-rounded villain is responsible for giving Barry his speed (as well as other metahumans their powers) in addition to causing the hero a great deal of pain by murdering our hero's mother when he was a child in a twisted attempt (one of many) to improve Barry's heroics.
What gave Thawne his many layers in the premiere season was his decision to pose as scientist Harrison Wells and become Barry's mentor. Initially born out of self-serving pragmatism, Thawne's time with the budding speedster led to the former beginning to care for Barry despite his intense hatred for the latter.
The show was lucky to have Tom Cavanagh inhabit the role due to the actor's effortless ability to switch between the level-headed Wells and the unhinged Thawne and still give depth to both personas.
It is a shame that later seasons chipped at Eobard's menace by having him make pointless appearances, but they have done little to ruin the formidable presence he brought to the first season.