When Arrow debuted in 2012, its fans marvelled (no pun intended) at DC's decision to go dark, gritty, and above all - realistic. That all went out the window with the introduction of The Flash in 2014, when the Arrowverse was born, and with it came metahumans, aliens, time-travel and magic.
Somehow Arrow survived, and both shows continue to thrive, with the Legends of Tomorrow and even Supergirl recently getting in on the action - but you can't have that many superheroes without a few equally supervillains.
For every season of Arrow, Flash and Legends (I'm excluding Supergirl villains on the grounds that she only recently became a part of the Arrowverse), they have faced a new enemy, a 'big bad' if you will - suitably bigger and badder than their predecessors.
But this would be no list at all if I were ranking simply on size and evilness. Instead, the villains are ranked on their respective story arcs; how much impact they have had on our protagonists, and their own backstories and justifications.