10. The Hotness
"II don't think so, copper. Not until I get my money!"
Words that will on in infamy within the confines of The Flash fandom, The Hotness holds the undisputed title of "most one-dimensional villain" to ever surface in the Arrowverse. It's not because he has fire powers or because he was a villain-of-the-week, it's simply down to the fact that there was one thing driving him to do what he did: Money. And we were going to hear about it.
A man who accidentally found himself being given fire powers that weren't his, he decided to go out and rob a bank and frighten people because, well, we don't know. From his corny one-liners (about money, in case you didn't know) to his lack of motivations, the character is widely regarded as one of the most pointless in The Flash's nine-season run. Yes, we know Iris needed a fiery threat to showcase her powers in that episode, but surely we could have given her something beyond these basic bad guy motivations.
The Hotness returned in seasons 8 and 9 and fared a bit better because the writers did some damage control and gave him the beginnings of a backstory. Turns out he's a pretty great guy named Jaco Birch with a son who he wants to provide for. See how he instantly became 100000000000 times more interesting?
His emotional backstory didn't suit the narrative at the time of his first appearance so he was given absolutely nothing.
But hey, we'll always have the moneyyyyyyyyy.